Allow appending to User-Agent headers
Allow appending to User-Agent headers

Before this commit, `local_agent` could be used to *prepend* a string to
the User-Agent header. There was no way to append.

This commit adds explicit `local_agent_prepend` and `local_agent_append`
which do what you would expect. The original method is left in place for
compatibility reasons.

rusoto/rusoto,rusoto/rusoto,DualSpark/rust-aws,DualSpark/rust-aws,DualSpark/rust-aws,DualSpark/rusoto,rusoto/rusoto,DualSpark/rusoto,DualSpark/rusoto