Allow configuring MellonSignatureMethod as of cosmic

[Dmitrii Shcherbakov]

Some IdPs require specific signature methods to be set.

Mellon versions up to 0.13.1 (bionic) did not expose MellonSignatureMethod and the default that comes from lasso is LASSO_SIGNATURE_METHOD_RSA_SHA1.

As of 0.14.0 (Cosmic) MellonSignatureMethod is exposed and is set by default to LASSO_SIGNATURE_METHOD_RSA_SHA256.

https://github.com/Uninett/mod_auth_mellon/releases/tag/v0.14.0

We should enable this option in case an IdP requires something else (e.g. sha384, sha512).

With ADFS as an IdP its operator has to explicitly choose which signature method to use and in contemporary versions of ADFS it is set to sha256 which leads to IdP-side errors and mellon <=0.13.1