feat(admin): add safe OIDC provider mutations

[cxymds]

Summary

• add rc admin idp openid set, update, enable, and disable
• merge updates with the current provider so omitted fields and existing secrets are preserved
• accept replacement secrets only from stdin or protected regular files with explicit acknowledgement
• validate with the server before mutation and emit deterministic redacted diffs
• support dry-run/no-op paths that never send PUT, plus typed bounded sensitive responses and output-v3

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• OIDC integration tests: 14 passed
• git diff --check

Closes rustfs/backlog#1490
Refs rustfs/backlog#1379

BREAKING: no