feat: Add session tag in key-value format

[Yaminyam]

close #390
The session tag function, which previously saved text like a regular memo, can now be saved in key-value format.
The key is limited to a maximum of 128 characters, and the value is limited to a maximum of 256 characters.
Input requires the form 'key1=value1, key2=value2, ...'.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[fregataa]

How about using(or updating) ai.backend.cli.params.CommaSeparatedKVListParamType?

[fregataa]

We have customized column schemas in ai.backend.manager.models.base.
It will be good for performance to set a constraint in SQL but how about making(or updating) a column schema?
ai.backend.manager.models.base.StructuredJSONColumn would work but it might need update.