Enterprise jsonnet: add config to create tokengen job explicitly

[yvrhdn]

What this PR does:
The jsonnet for GET creates a job to generate the token from a GET license (tokengen). This job should be created once when installing GET and the job can not be modified afterwards. To make this easier to manage, this PR adds a config option run_tokengen_job which is disabled by default and can be set to explicitly create the tokengen job.

How it works:

The first time a GET cluster is created the user should set:

  _config+:: {
    run_tokengen_job: true,
  },

During tk apply, tanka will create the tokengen job (just like before). After the job has run, the user should remove this line and the tokengen job will not be modified anymore by tanka.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added: update https://grafana.com/docs/enterprise-traces/next/setup/tanka/
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[hedss]

Perfect! Thank you for doing this!