fix(oidc): store requested response_mode (#8145)

# Which Problems Are Solved

Zitadel never stored or returned the requested `response_mode` in oidc
Auth Requests. This caused the oidc library to fallback to the default
based on the response_type.

# How the Problems Are Solved

- Store the `response_mode` in the Auth request repo
- Store the `response_mode` in the Auth request v2 events
- Return the `resonse_mode` from the Auth Request v1 and v2
`ResponseMode()` methods. (Was hard-coded to an empty string)

# Additional Changes

- Populate the `response_modes_supported` to the oidc Discovery
Configuration. When it was empty, the standard specifies the default of
`query` and `fragment`. However, our oidc library also supports
`form_post` and by this fix, zitadel now also supports this.

# Additional Context

- Closes #6586
- Reported
https://discord.com/channels/927474939156643850/1151508313717084220

---------

Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/api/oidc/server_test.go

@@ -73,7 +73,7 @@ func TestServer_createDiscoveryConfig(t *testing.T) {
 				RegistrationEndpoint:                               "",
 				ScopesSupported:                                    []string{oidc.ScopeOpenID, oidc.ScopeProfile, oidc.ScopeEmail, oidc.ScopePhone, oidc.ScopeAddress, oidc.ScopeOfflineAccess},
 				ResponseTypesSupported:                             []string{string(oidc.ResponseTypeCode), string(oidc.ResponseTypeIDTokenOnly), string(oidc.ResponseTypeIDToken)},
-				ResponseModesSupported:                             nil,
+				ResponseModesSupported:                             []string{string(oidc.ResponseModeQuery), string(oidc.ResponseModeFragment), string(oidc.ResponseModeFormPost)},
 				GrantTypesSupported:                                []oidc.GrantType{oidc.GrantTypeCode, oidc.GrantTypeImplicit, oidc.GrantTypeRefreshToken, oidc.GrantTypeBearer},
 				ACRValuesSupported:                                 nil,
 				SubjectTypesSupported:                              []string{"public"},