feat: experimental CORS proxy for OIDC

[csmig]

The CORS proxy feature is experimental for use by deployments unable to control their OIDC Provider's CORS policy. The web client (UI) is configured to use the CORS proxy by setting:

STIGMAN_CLIENT_OIDC_PROVIDER = "api/op/cors-proxy/oidc"

The CORS proxy forwards incoming requests to the OIDC Provider configured by STIGMAN_OIDC_PROVIDER

Adds endpoints:

/op/cors-proxy/oidc/.well-known/openid-configuration
/op/cors-proxy/oidc/token

Adds optional envvar:
STIGMAN_OIDC_PROXY_HOST: No default The Host: header value to be used by the CORS proxy for outbound requests. Some OIDC providers return configuration metadata with endpoint URLs having this value as their base.