Custom user agent string

[kommendorkapten]

It would be good during debugging sessions if the user agent string contained the go-tuf version instead of the default Go user agent string. Also, during tuf client initializeation the client should be able to append their name, so the string becomes something like: go-tuf/vx.y cosign/vx.y.

cc @jku

[jku]

see also
sigstore/sigstore-go#143
theupdateframework/python-tuf#2611