user agent is not actually being modified

[mattsb42-aws]

Problem

I could have sworn I checked this, but apparently the user agent modification we are doing is not actually taking effect.

Solution

It looks like this needs to be modified on the botocore level, not on the boto3 client level. Unfortunately, this means that we cannot modify the user agent if a user provides a custom KMS client, as we should not be modifying a botocore client that might be used for something else.

In KMSMasterKeyProvider we can modify the botocore client if we build it directly. However, we should not modify a botocore client that was provided to us.

TODO

Make sure that there is not a way to modify the user agent at the boto3 client layer.

[mattsb42-aws]

As part of this, I think we should be copying the botocore session provided rather than using it directly.