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/pubspec.yaml

@@ -0,0 +1,34 @@
+name: halkyon_oidc_server
+description: >-
+  Signing in with OpenID Connect for Serverpod, as a module: any provider
+  that follows OpenID Connect's code flow, SimpleLogin a preset. Keeps only
+  the provider's ID of a person; the ID token checked against the provider's
+  keys, the nonce and the attempt the device holds.
+version: 0.1.0
+# In the Halkyon repository until it runs on learn.halkyon.eu (ADR 0023, D3).
+publish_to: none
+
+environment:
+  sdk: ^3.13.0
+
+resolution: workspace
+
+dependencies:
+  clock: ^1.1.3
+  # The PKCE challenge's SHA-256.
+  crypto: ^3.0.7
+  # Checks ID tokens' signatures (ADR 0020, D12). Pinned exactly, as what checks
+  # who someone is must never move by accident; the version Serverpod's own
+  # sign-ins resolve to (^3.1.0).
+  dart_jsonwebtoken: 3.4.1
+  # The providers' discovery documents, keys and token endpoints.
+  http: ^1.6.0
+  meta: ^1.16.0
+  serverpod: 4.0.4
+  serverpod_auth_idp_server: 4.0.4
+
+dev_dependencies:
+  # Makes the RSA keys the tests sign their providers' tokens with.
+  pointycastle: ^4.0.0
+  serverpod_test: 4.0.4
+  test: ^1.25.5