Move UserAgent to http-remoting-api

[ash211]

@uschi2000 can you please tag this with an API break label? When we go to remoting4 I'd like to be able to use this class in the API of our foundry-spark library.

See also https://github.com/palantir/http-remoting-api/issues/59

[stale]

This issue has been automatically marked as stale because it has not been touched in the last 60 days. Please comment if you'd like to keep it open, otherwise it'll be closed in 7 days time.

[iamdanfox]

This got implemented when we moved http-remoting-api -> conjure-java-runtime-api: https://github.com/palantir/conjure-java-runtime-api