Add ENCRYPTION_KEYPAIRS to settings

[gregorywong]

This PR introduces a new configuration variable ENCRYPTION_KEYPAIRS to the settings.
In addition to allowing the user to set it, this enhancement will automatically set it for them if both KEY_FILE and CERT_FILE are defined.

When handling encrypted assertions, encryption_keypairs must be set in order for saml2 to properly decrypt them. Unfortunately, this is not fully documented, but it is a known issue noted by other users of pysaml2 (or libraries requiring it):
IdentityPython/pysaml2#346
IdentityPython/djangosaml2#22

This enhancement ensures that encrypted assertions can be handled successfully, while keeping the configuration settings DRY (i.e., no need to include CERT_FILE and KEY_FILE twice).

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[gregorywong]

Tagging @mostafa for review. Thanks!

[mostafa]

@gregorywong
Thanks for your contribution! LGTM, but please consider fixing the comment before I merge.

[mostafa]

🎉