Use bearer tokens for third-party-transfer
With this work, the client can include a source and destination token issuer in the file-metadata.
When this is included, FTS3 will try to use libX509SciTokensIssuer (as from https://github.com/bbockelm/x509-scitokens-issuer) to generate a bearer token from a X509 certificate.
If successful, FTS3 will then configure GFAL2 to utilize the bearer token instead of the GSI proxy (see dmc/gfal2!8 (merged)).
Overall, this allows one to do HTTP-based third-party-copy without having to contact either endpoint using GSI.