API versioning, deprecation, and `Sunset` for breaking stream contract changes

[greatest0fallt1me]

Versioning policy, Deprecation and Sunset headers, and v2 migration

    ## Description

    As **streams** and **escrow** fields evolve, define **/v1** and **/v2** (or **header

versioning**), 90-day minimum deprecation, Deprecation and optional Sunset
headers, and a public changelog for wallet partners. This reduces surprise
breakages in Soroban-related integrations.

    ## Requirements and context

    - **Changelog** in `CHANGELOG.md` and linked from README.

• OpenAPI tags per version; no silent field removal in minor if semver applies.

• CI test that v1 still passes contract tests for N months.

• Comms template for major deprecations.

• Error with link to migration guide when v1 is called post-sunset.

    ## Suggested execution
  
    1. `git checkout -b feature/api-sunset-v1`
  

2. Add version router; duplicate handlers only where needed (prefer feature flags).

3. PR with migration doc for 3 breaking changes (examples).

4. Security N/A; DX primary.

5. 96h for framework; per-field moves can be child issues.

    ## Test and commit
   

• Run the full test suite; add or update tests until the agreed coverage bar is met.
• Cover edge cases listed in this issue; document any intentional exclusions with brief rationale in the PR.
• Include relevant test output (e.g. test runner summary) or a link to a passing CI run in the pull request.
• Add security notes for auth, keys, PII, chain settlement, or money movement (assumptions verified, out-of-scope items).

Example commit message

docs(api): add versioning, Sunset headers, and migration guide for stream API changes

Guidelines

• Target: at least 95% coverage on new or meaningfully changed code (per the repo’s standard tooling).
• Documentation: update contributor-facing or API documentation where a reviewer would be blocked without it.
• Timeframe: 96 hours to ready-for-review (surface blockers early).