[DOP-28218] Split GET /v1/tags and /v1/tag-values

[dolfinus]

Change Summary

Having list of tags with nested list of values is okay for limited scope (get only tag values bound to dataset/job/etc). But it's not very convenient for working with unbound list of values - e.g. getting all list of tags in UI will load all existing tag values, even if it's not actually used.

Instead of introducing paginagion of tag values into already paginated tags list. split GET /v1/tags into 2 endpoints - GET /v1/tags returns only tag names (paginated), and GET /v1/tag-values returns only tag values (paginated).

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
data_rentgen/server/services
dataset.py	27	1	96%	36
job.py	30	1	96%	37
tag.py	31	1	96%	44
tag_value.py	23	1	95%	32
TOTAL	7645	924	87%

[dolfinus]

It looks like UI doesn't actually need this: MobileTeleSystems/data-rentgen-ui#123