Support x5t in place of kid

[szh]

Enhancement Request

Some JWTs, including those generated by Azure DevOps, do not contain a kid field and instead use the x5t field: https://www.rfc-editor.org/rfc/rfc7515#section-4.1.7

From a cursory look through the code in this library, it seems that this should require some changes in the key_finder.rb file.