feat: Add OpenStack Keystone authentication service

[benroeder]

Summary

• Add reqsign-openstack-keystone crate implementing Keystone v3 password authentication
• Wire into the reqsign umbrella crate behind the openstack feature flag
• Add CI workflow with mock server integration tests

This enables OpenDAL's Swift backend to use reqsign for credential management instead of inline Keystone auth code. Contributes to #685 (reqsign-swift checklist item).

Context: Xuanwo requested this move during review of apache/opendal#7216.

What's included

Credential types (credential.rs):

• Credential with token, expiry, and service catalog
• CatalogEntry / Endpoint types with serde deserialization
• endpoint() and endpoint_in_region() for catalog lookup
• SigningCredential impl with 2-minute expiry grace period

Request signing (sign_request.rs):

• RequestSigner inserts X-Auth-Token header (marked sensitive)

Credential providers (provide_credential/):

• KeystoneCredentialProvider — POST to {auth_url}/auth/tokens, extracts token from X-Subject-Token header, parses expiry + service catalog
• EnvCredentialProvider — reads OPENSTACK_AUTH_URL, OPENSTACK_USERNAME, OPENSTACK_PASSWORD, etc.
• DefaultCredentialProvider — credential chain (env vars for now)

Testing (27 tests total):

• 16 unit tests covering credential validity, catalog lookup, region filtering, edge cases, serde, auth request building
• 11 integration tests with Python mock Keystone server: scoped/unscoped tokens, bad credentials, connection refused, full Signer round-trip with token caching, region filtering
• Also tested against a production OpenStack Keystone v3 deployment — confirmed token acquisition, timestamp parsing, and service catalog parsing with multiple services and regions

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --no-fail-fast (all 27 openstack-keystone tests + full workspace green)
• Mock server integration tests via CI workflow
• Verified against production Keystone v3 cluster