* feat: choose signing cert explicitly in saml metadata
when parsing, we want to explicitly choose the use='signing' cert instead of accidentally choosing the encryption one
ENT-677
* test: test for parse_metadata_xml
* feat: also address case when signing key is absent
in this case we consider the 'use'-less keydescriptior as if it's for both signing and encryption
* test: test case for missing use=signing attribute
* test: fix test failure
33437e7fef