[ENHANCE] Allow generateServiceProviderMetadata to discover the encryption capabilities of the underlying system

[alexstuart]

generateServiceProviderMetadata includes a static list of algorithms that are encoded into EncryptionMethod elements.

Note that #13 discovers that the static list is incorrect and proposes replacing the algorithms with correct URIs.

A more robust solution would be for generateServiceProviderMetadata to query the system and discover the supported algorithms, which are subsequently published in metadata. Such a solution is significantly more work than fixing two typos so I've submitted this issue as an enhancement.

[tomgiddingsjisc]

Just to add to Alex's suggestion (thanks Alex!) - I'm more than happy to have a look at this and create a PR if it's welcome.

This could be along the lines of:

1. Adding getEncryptionAlgorithms as part of algorithms.ts.
2. Creating a new encryptionAlgorithm core option and type to allow an array of the above options.
3. Use the generated array to replace the existing fixed array, with the original options being supplied as a default if encryptionAlgorithm isn't set.
4. Followup PR for passport-saml on release.

Though obviously please do let me know if you would prefer a different approach, or have any suggestions.

[cjbarth]

I'd be interested in seeing a PR to accomplish this. I'm not sure of the entire advantage since we know what the underlying library supports and can statically respond with that. However, such an enhancement would allow us to not worry about dependency changes.