Custom user agent string

[kommendorkapten]

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

cc https://github.com/theupdateframework/go-tuf/issues/630

[jku]

tuf-js/vx.y npm-cli/vx.y

a detail but I would suggest npm-cli/x.y tuf-js/x.y (or just npm-cli/x.y)

• most important component first
• no v-prefix for version number