feat: support multiple OIDC redirect URIs (#25408)

This PR adds a new opt-in setting, `CODER_OIDC_REDIRECT_ALLOWED_HOSTS`,
that lets a single Coder deployment complete OIDC login on more than one
hostname. When the allowlist is non-empty, Coder picks the OIDC
`redirect_uri` based on the incoming request's Host header (validated
against the list) instead of always using the static URL derived from
`CODER_ACCESS_URL`. When unset, the (default) behavior is identical to
today.

The motivation is that a single Coder deployment is frequently reachable
via multiple hostnames - for example, an internal hostname for users on
a corporate VPN and a different hostname routed through a zero-trust
gateway for users off-VPN - but OIDC login today only works on whichever
single hostname `CODER_ACCESS_URL` points to, because the `redirect_uri`
sent to the IdP is fixed at server startup. Users who reach the
deployment on any other valid hostname can see the login page but fail
the OIDC callback, since the IdP redirects them back to a hostname they
can't reach (or whose cookies they don't have).

cli/testdata/server-config.yaml.golden

@@ -448,6 +448,13 @@ oidc:
   # enable if you understand and accept the risk.
   # (default: <unset>, type: bool)
   dangerousOidcEmailFallback: false
+  # An allowlist of hostnames that may be used as the host of the OIDC redirect_uri.
+  # When set, the redirect_uri sent to the OIDC provider is built from the incoming
+  # request's Host header (validated against this list) instead of from access-url.
+  # Every listed host must also be registered as a valid redirect URI in the OIDC
+  # provider. Ignored when oidc-redirect-url is set.
+  # (default: <unset>, type: string-array)
+  oidcRedirectAllowedHosts: []
 # Telemetry is critical to our ability to improve Coder. We strip all personal
 #  information before sending data to our servers. Please only disable telemetry
 #  when required by your organization's security policy.