Introducing changelog-gen (#8387)

* add templates for changelog-gen
* add entry files for currently unreleased PRs on master

.changelog/7628.txt

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+```release-note:improvement
+agent: Allow to restrict servers that can join a given Serf Consul cluster.
+```

.changelog/7899.txt

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+```release-note:improvement
+acl: allow auth methods created in the primary datacenter to optionally create global tokens.
+```

.changelog/7970.txt

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+```release-note:feature
+acl: Added ACL Node Identities for easier creation of Consul Agent tokens.
+```

.changelog/8158.txt

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+```release-note:bug
+connect: fix crash that would result if a mesh or terminating gateway's upstream has a hostname as an address and no healthy service instances available.
+```

.changelog/8190.txt

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+```release-note:improvement
+connect: Append port number to expected ingress hosts.
+```

.changelog/8194.txt

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+```release-note:improvement
+connect: various changes to make namespaces for intentions work more like for other subsystems.
+```

.changelog/8211.txt

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+```release-note:bug
+agent: Fixed a bug where Consul could crash when `verify_outgoing` was set to true but no client certificate was used.
+```
+
+```release-note:bug
+auto_encrypt: Fixed an issue where auto encrypt certificate signing wasn't using the connect signing rate limiter.
+```
+
+```release-note:bug
+auto_encrypt: Fixed several issues around retrieving the first TLS certificate where it would have the wrong CN and SANs. This was being masked by a second bug (also fixed) causing that certificate to immediately be discarded with a second certificate request being made afterwards.
+```

.changelog/8216.txt

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+```release-note:improvement
+connect: support Envoy v1.14.4, v1.13.4, v1.12.6.
+```

.changelog/8218.txt

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+```release-note:improvement
+dns: Improve RCODE of response when query targets a non-existent datacenter. [[GH-8102](https://github.com/hashicorp/consul/issues/8102)]
+```

.changelog/8222.txt

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+```release-note:bug
+xds: version sniff envoy and switch regular expressions from 'regex' to 'safe_regex' on newer envoy versions.
+```

.changelog/8268.txt

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+```release-note:improvement
+version: The `version` CLI subcommand was altered to always show the git revision the binary was built from on the second line of output. Additionally the command gained a `-format` flag with the option now of outputting the version information in JSON form. **NOTE** This change has the potential to break any parsing done by users of the `version` commands output. In many cases nothing will need to be done but it is possible depending on how the output is parsed.
+```

.changelog/8311.txt

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+```release-note:bug
+auto_encrypt: Fixed an issue that caused auto encrypt certificates to not be updated properly if the agents token was changed and the old token was deleted.
+```

.changelog/8343.txt

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+```release-note:bug
+gossip: Avoid issue where two unique leave events for the same node could lead to infinite rebroadcast storms.
+```

.changelog/8371.txt

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+```release-note:bug
+connect: Fixed issue where specifying a prometheus bind address would cause ingress gateways to fail to start up.
+```

.changelog/changelog.tmpl

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+{{- if index .NotesByType "breaking-change" -}}
+BREAKING CHANGES:
+
+{{range index .NotesByType "breaking-change" -}}
+* {{ template "note" .}}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.security }}
+SECURITY:
+
+{{range .NotesByType.security -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.feature -}}
+FEATURES:
+
+{{range .NotesByType.feature -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.improvement }}
+IMPROVEMENTS:
+
+{{range .NotesByType.improvement -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.bug }}
+BUG FIXES:
+
+{{range .NotesByType.bug -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+

.changelog/note.tmpl

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+{{- define "note" -}}
+{{.Body}} [[GH-{{- .Issue -}}](https://github.com/hashicorp/consul/pull/{{- .Issue -}})]
+{{- end -}}