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>

docs/adr/0023-openid-connect-module.md

@@ -60,8 +60,9 @@ as well at their convenience with Serverpod", what Serverpod v4.0.4 offers:
    and the nonce, 32 random bytes each, and PKCE's verifier where used; who began it,
    if signed in; ten minutes, taken once. The host may begin the state with a marker of
    its own (ADR 0020's forwarding page reads one).
-5. **Checking a sign-in** (ADR 0020's D12): the code exchanged with the client's secret
-   (`OAuth2PkceUtil`); the ID token's header without `typ` asked for, its algorithm one
+5. **Checking a sign-in** (ADR 0020's D12): the code exchanged with the client's secret,
+   by the module itself rather than `OAuth2PkceUtil`, so that a provider out of reach (no
+   answer in time, a 5xx) is told apart from a refused code; the ID token's header without `typ` asked for, its algorithm one
    of the provider's, its key the one its `kid` names in the provider's keys (kept an
    hour, fetched again for an unknown `kid` at most once a minute, every fetch with a
    timeout); the issuer, the audience (and `azp` with more than one), the expiry and the