feat(oidc): a Serverpod module for signing in with OpenID Connect, SimpleLogin a preset (ADR 0023)

halkyon_oidc_server and its generated client halkyon_oidc_client: any
provider following OpenID Connect's code flow, beside Serverpod's own
providers, with a preset for SimpleLogin (no PKCE, no `typ`). Nothing
of Halkyon's is in it; BSD-3-Clause, `publish_to: none` for now.

- Attempts in `halkyon_oidc_attempt` (ADR 0020's D13): the ID
  PostgreSQL's gen_random_uuid(), a state and a nonce of 32 random
  bytes, PKCE's verifier where used, the caller if signed in; ten
  minutes, taken once. A host may begin the state with a marker.
- The code exchanged by the module itself, so a provider out of reach
  (a timeout, a 5xx) is told apart from a refused code; the ID token
  checked with dart_jsonwebtoken's algorithms (ADR 0020's D12): the
  provider's algorithms alone, the key its `kid` names, of the right
  type; issuer, audience and `azp`, expiry, time issued, `nbf`, a
  minute's leeway; the attempt's nonce; a `sub`.
- Keys kept an hour, fetched again for an unknown `kid` at most once
  a minute; discovery checked against the issuer; every request with
  a timeout.
- Identities in `halkyon_oidc_identity`: the provider's key and the
  `sub`, one per person and provider, deleted with the person.
- The stock endpoint (`OidcIdpBaseEndpoint`), and the parts for a host
  with rules of its own: begin, verify, keep and takeKept, link,
  accountOf, unlink; merging people; a limit per network.
- The forwarding page, moved here from halkyon_base in the next commit.

The package's main library is the code generator's, as halkyon_base's;
a host imports `package:halkyon_oidc_server/oidc.dart`. ADR 0023's
point 5 no longer names OAuth2PkceUtil, which the module doesn't use.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

pubspec.yaml

@@ -24,6 +24,8 @@ workspace:
   - packages/halkyon_core
   - packages/halkyon_interop
   - packages/halkyon_keys
+  - packages/halkyon_oidc_client
+  - packages/halkyon_oidc_server
   - packages/halkyon_ui
   - packages/halkyon_ui/example