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>

packages/halkyon_oidc_server/analysis_options.yaml

@@ -0,0 +1,22 @@
+# The workspace's lint rules (see ../../analysis_options.yaml), minus generated code.
+include: ../../analysis_options.yaml
+
+analyzer:
+  exclude:
+    - lib/src/generated/**
+    - test/integration/test_tools/serverpod_test_tools.dart
+
+# Serverpod's code generator reads this package with an analyzer that doesn't know
+# Dart v3.13's primary constructors yet, and fails on them in endpoint files ("requires
+# the 'primary-constructors' language feature"; checked with v4.0.4). So the module
+# keeps the classic constructor and class-body syntax, as the Halkyon servers do;
+# switch these back on once Serverpod parses it.
+linter:
+  rules:
+    empty_container_bodies: false
+    unnecessary_type_name_in_constructor: false
+    use_declaring_parameters: false
+    use_primary_constructors: false
+
+formatter:
+  trailing_commas: preserve