Add support for entry attachments

[atimin]

Closes #98

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• CHANGELOG.md has been updated (for bug fixes / features / docs)

What kind of change does this PR introduce?

Feature

What was changed?

• Added entry attachment APIs to IBucket:
  • WriteAttachments(entry_name, attachments)
  • ReadAttachments(entry_name)
  • RemoveAttachments(entry_name, attachment_keys)
• Implemented attachment storage using <entry_name>/$meta records with a key label.
• Added JSON validation for attachment payloads on write/read paths.
• Added integration tests for write/read, selective removal, and full removal of attachments ([entry_api][1_19]).
• Updated CI stable-server test exclusion tag from ~[1_18] to ~[1_19].
• Added an unreleased changelog entry for the new API.

Related issues

• Support for Entry Attachments #98

Does this PR introduce a breaking change?

No.

Other information:

• Feature is intended for ReductStore API v1.19+.