[v2] User agent 2.1: Track client features

[hssyoo]

Ports boto/botocore#3389 and boto/s3transfer#335

I also added a CLI-specific change to wrap the CLI entrypoint under the start_as_current_context context manager so future work can add CLI-specific features.

> aws s3 cp myfile s3://mybucket
> aws history show

...
"User-Agent": "aws-cli/2.24.14 ... m/G cfg/retry-mode#standard ..."
...

[ashovlin]

Changes look good. For the S3 high-level commands, can we track the HeadObject calls too (unless the internal design says not to)?

[hssyoo]

Changes look good. For the S3 high-level commands, can we track the HeadObject calls too (unless the internal design says not to)?

Addressed in 35e6716

I considered only registering at the point of HeadObject/ListObjectsV2 calls (code), but decided to just move it under S3TransferCommand so that we consider any API calls made in this class of commands to have used S3Transfer.