feat(openstack): Add Keystone v3 authentication support

[benroeder]

Summary

• Add services-openstack feature with Keystone v3 password authentication
• OpenstackConfig — loads auth_url, username, password, domain/project
  settings from env vars or fields
• OpenstackLoader — authenticates against Keystone, caches tokens with
  2-minute expiry grace, parses service catalog
• OpenstackCredential — token + expiry + service catalog with endpoint
  lookup by service type, interface, and region
• OpenstackSigner — signs requests by inserting X-Auth-Token header
• 15 unit tests covering credential validity, catalog lookup, serde,
  and request signing
• Add services-openstack to CI build_single_feature matrix

Context

Companion to #690 (main branch). This backport targets v0.16.x which
is the version OpenDAL currently depends on. Tested against a real
OpenStack Keystone cluster.

Related: #685

Files

New (790 lines):

• src/openstack/mod.rs — module re-exports
• src/openstack/config.rs — Config + from_env()
• src/openstack/credential.rs — Credential, Loader, CatalogEntry, Endpoint
• src/openstack/signer.rs — Signer

Modified:

• Cargo.toml — add services-openstack feature
• src/lib.rs — add feature-gated module
• .github/workflows/ci.yml — add to build matrix

Test plan

• cargo fmt --all -- --check passes
• cargo test --no-fail-fast — 81 unit + 32 integration + 7 doc-tests all pass
• cargo check --all-features compiles clean (only pre-existing warnings)
• Tested against real OpenStack Keystone cluster

[Xuanwo]

Thank you! But we don't need to backport to v0.16 for new features.