Allow the user header agent to be set for the tuf client.

[kommendorkapten]

Description

Similar to #52 but for the tuf client. This work depends on theupdateframework/tuf-js#681

[jku]

I'm trying to collect some client statistics and having a sigstore-js (or higher level application) user-agent would be really nice at this point: sigstore/rekor-tiles#480