feat(env): add customapi group with list, create, generate-openapi

[zekelinAlex]

Adds a customapi noun group under txc env for working with Custom APIs in the connected environment.

Three commands: list shows registered Custom APIs with optional --search filtering, create registers a new Custom API including request parameters and response properties, and generate-openapi emits an OpenAPI 3.0 spec (JSON) for one or all Custom APIs, either to stdout or a file via --output.

While wiring up create through the changeset pipeline I noticed ChangesetApplier was building entities without attribute metadata, so typed values (optionsets, money, lookups) went to Dataverse as raw JSON primitives. It now prefetches entity metadata for record CREATE/UPDATE ops and converts values the same way the direct record service does.

Closes #98