Streamline identities CRUD + data model improvements

[khvn26]

DoD

• edge-identities APIs are deprecated.
• Identities management API and UI are the same for self-hosted and SaaS. That includes override counters and identity overrides in the feature flyout.
• Identity overrides are written to environments_v2 table, after Core, not instead of it.
• Segments are written the same way to the environments_v2 table as well.
• Trait updates are synced back to Core.

Notes

Internally, segment and identity overrides could be stored simply as segment overrides. It's achievable after #5674 is done using the $.identity.identifier property in conjunction with the IN operator.

[khvn26]

Needs more scoping. Should be broken down into following:

Items essential to parent epic:

• Segment document design. Should consider indexable segment metadata, and non-indexed fields compression.
• Segment document persistence: one-way sync to Dynamo.

Backlog items:

• Compressing identity overrides to segments on persistence.
• Postgres-based Identities data design
• Core DB access for the Lambda extensions + trait sync.
• edge-identities API deprecation.
• Migrate counter badges and environment statistics to Core storage.