Rewrite LabelAPIClient to be able to return error messages from Label
Repo API.
Main features:
1. Raises LabelRepoAPIException when response code is 400 or 500 level.
2. Always return response as a second argument to further inspect it, if
necessary.
# What this PR does
Adds labels implementation for integrations:
- ability to create/update labels on creating/updating integration
- ability to associate labels to integrations
- cache for label reprs on OnCall side
- feature flag to enable/disable labels
## Which issue(s) this PR fixes
https://github.com/grafana/oncall-private/issues/2157
## Checklist
- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)
---------
Co-authored-by: Maxim <maxim.mordasov@grafana.com>
Co-authored-by: Rares Mardare <rares.mardare@grafana.com>
