generic API for modifying headers

[rcoh]

see #583

There should be a generic way to add headers to request

[rcoh]

key-val support for adding to the user agent

[marcbowes]

We (qldb) are just about ready to ship our 2.0 shell based on the awssdk for Rust. At this point, setting the user agent is the only blocking issue in terms of feature parity. If you give us some pointers, we can probably contribute this.

[rcoh]

Because you’re already using the low level API, you can actually do this directly with no code changes on the SDK side, just use properties_mut() on the operation to set a custom AwsUserAgent

[rcoh]

we may need some very small changes depending on exactly what you want the user agent to be, but those should be quite minor and isolated

[rcoh]

A workable pattern for this:
https://gist.github.com/rcoh/14668328600aa8989b679e42ee92d45f

[jdisanti]

RFC for this feature

[jdisanti]

Resolving this in favor of #1112. Looks like it got duplicated.