Use FIPS endpoints when defined in ~/.aws/config or AWS_USE_FIPS_ENDPOINT environment variable

To simplify the use of FIPS endpoints I recommend that git-remote-codecommit use the same mechanisms as the AWS SDKs:

https://docs.aws.amazon.com/general/latest/gr/rande.html#FIPS-endpoints

> To specify a FIPS endpoint when you call an AWS operation, use a mechanism provided by the tool that you're using to make the call. For example, the AWS SDKs provide the following mechanisms to enable the use of FIPS endpoints:
>
> - Set the AWS_USE_FIPS_ENDPOINT environment variable to true
> - Add use_fips_endpoint=true to your ~/.aws/config file

We currently use the undocumented environment variable CODE_COMMIT_ENDPOINT to do this. But would prefer that the AWS_USE_FIPS_ENDPOINT apply here also.