Set the AuthnRequestsSigned attribute in the Saml2 metadata filter

[xpmatteo]

Previously, the AuthnRequestsSigned attribute was not set in the
metadata xml that is produced by the Saml2 metadata filter. As a
result, the maintainers of the IDP I had to register with were rejecting
our application.

Now, the attribute is always set, namely to "true" if the asserting
party wants signed authentication requests, and false otherwise. This
declares to the asserting party whether authentication requests will be
signed or not.

[jzheaux]

Hi, @xpmatteo. Thanks for the contribution.

While I see your intent, I don't think we want to assume RP metadata based on AP metadata. Whether or not an RP will sign AuthnRequests stands somewhat independent from whether or not an AP wants them signed.

Instead, what about adding a Converter method that allows the EntityDescriptor to be edited before getting serialized, e.g. setEntityDescriptorCustomizer(Consumer<EntityDescriptorParameters> entityDescriptor)? This would be similar to OpenSaml4AuthenticationRequestResolver#setAuthnRequestCustomizer.

[jzheaux]

Closed in favor of #10839