fix(saml): improve error handling (#8928)

# Which Problems Are Solved

There are multiple issues with the metadata and error handling of SAML:
- When providing a SAML metadata for an IdP, which cannot be processed,
the error will only be noticed once a user tries to use the IdP.
- Parsing for metadata with any other encoding than UTF-8 fails.
- Metadata containing an enclosing EntitiesDescriptor around
EntityDescriptor cannot be parsed.
- Metadata's `validUntil` value is always set to 48 hours, which causes
issues on external providers, if processed from a manual down/upload.
- If a SAML response cannot be parsed, only a generic "Authentication
failed" error is returned, the cause is hidden to the user and also to
actions.

# How the Problems Are Solved

- Return parsing errors after create / update and retrieval of an IdP in
the API.
- Prevent the creation and update of an IdP in case of a parsing
failure.
- Added decoders for encodings other than UTF-8 (including ASCII,
windows and ISO, [currently
supported](efd25daf28/encoding/ianaindex/ianaindex.go (L156)))
- Updated parsing to handle both `EntitiesDescriptor` and
`EntityDescriptor` as root element
- `validUntil` will automatically set to the certificate's expiration
time
- Unwrapped the hidden error to be returned. The Login UI will still
only provide a mostly generic error, but action can now access the
underlying error.

# Additional Changes

None

# Additional Context

reported by a customer

internal/idp/providers/saml/session.go

@@ -9,7 +9,6 @@ import (
 
 	"github.com/crewjam/saml"
 	"github.com/crewjam/saml/samlsp"
-	"github.com/zitadel/logging"
 
 	"github.com/zitadel/zitadel/internal/idp"
 	"github.com/zitadel/zitadel/internal/zerrors"
@@ -71,7 +70,7 @@ func (s *Session) FetchUser(ctx context.Context) (user idp.User, err error) {
 	if err != nil {
 		invalidRespErr := new(saml.InvalidResponseError)
 		if errors.As(err, &invalidRespErr) {
-			logging.WithError(invalidRespErr.PrivateErr).Info("invalid SAML response details")
+			return nil, zerrors.ThrowInvalidArgument(invalidRespErr.PrivateErr, "SAML-ajl3irfs", "Errors.Intent.ResponseInvalid")
 		}
 		return nil, zerrors.ThrowInvalidArgument(err, "SAML-nuo0vphhh9", "Errors.Intent.ResponseInvalid")
 	}