Google Provider Add Ttl from annotations

[stealthybox]

Use int64(ep.RecordTTL) in newRecord()
Fallback to hardcoded 300s for backwards-compat
Add TestNewRecords()
Add notes in ttl.md

[szuecs]

Much better than the hardcoded value 300, but do we have a default in the GoogleDNS zone already?
I thought about having no TTL set if there is no TTL specified, such that you can use the default in the origin configuration, which can be adjusted centrally, but this constant not.
I am fine with the PR, so we can also do it later, but it would be nice in general to have it set by the DNS zone itself, which is already the default TTL.
What do you think?

[stealthybox]

I don't know of a way to configure default TTL in the Cloud DNS zone. Do you know how to do this?

I tried omitting Ttl:

return &dns.ResourceRecordSet{
	Name:    ensureTrailingDot(ep.DNSName),
	Rrdatas: []string{target},
	// Ttl:     ttl,
	Type: ep.RecordType,
}

The resulting DNS records in gcloud have 0-value TTL's:

NAME                  TYPE  TTL    DATA
rightdns.com.         NS    21600  ns-cloud-b1.googledomains.com.,ns-cloud-b2.googledomains.com.,ns-cloud-b3.googledomains.com.,ns-cloud-b4.googledomains.com.
rightdns.com.         SOA   21600  ns-cloud-b1.googledomains.com. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
nginx.rightdns.com.   A     0      10.128.0.7
nginx.rightdns.com.   TXT   0      "heritage=external-dns,external-dns/owner=external-dns"
nginx2.rightdns.com.  A     0      10.128.0.7
nginx2.rightdns.com.  TXT   0      "heritage=external-dns,external-dns/owner=external-dns"

In the future, I believe we should be able to define an overrideable global default TTL for all endpoints in external-dns as well as min and max.
endpoint.TTL.isConfigured() would probably need to be refactored as well.

This is out of scope as it would affect all providers.

For now, I prefer the explicit behavior in this PR.
It matches the AWS provider implementation: {code}

[szuecs]

ACK. Thanks for your test and detailed comment!

[stealthybox]

Further discussion about this functionality in #390

[szuecs]

👍 @linki @ideahitme please merge, as discussed with @stealthybox in slack, I am fine with the current PR.

[stealthybox]

Added changelog -- thanks @szuecs

[ideahitme]

/lgtm