Add ability to map static values to metadata fields

[noahg1]

This proposed static_mapping would behave similarly to the claim_mapping field on the backend role, but rather than mapping a claim from the JWT to a metadata field the user would be able to specify what value they want mapped to the metadata field.

[dwimsey]

Added PR #346 for a quick implementation to do this. I welcome comments/suggestions on implementation options. This looked about like the simplest way.

[dwimsey]

If its considered are reasonable implementation, I can add tests and documentation