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_test.go

@@ -134,7 +134,7 @@ func TestSession_FetchUser(t *testing.T) {
 				requestID: "id-b22c90db88bf01d82ffb0a7b6fe25ac9fcb2c679",
 			},
 			want: want{
-				err: zerrors.ThrowInvalidArgument(nil, "SAML-nuo0vphhh9", "Errors.Intent.ResponseInvalid"),
+				err: zerrors.ThrowInvalidArgument(nil, "SAML-ajl3irfs", "Errors.Intent.ResponseInvalid"),
 			},
 		},
 		{