add to user-agent request header with env var data

[anGie44]

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Reference: hashicorp/terraform-provider-aws#9149

Please briefly describe the changes proposed in this pull request.

• In addition to the user-agent info stored in the Config object, we also want enable appending custom agent information set via an environment variable, namely, TF_APPEND_USER_AGENT.

[hashicorp-cla]

All committers have signed the CLA.

[anGie44]

is there any session unit testing that can be done to validate the request header?

[bflad]

Looks good to me 🚀 Regarding the unit testing, let's create a separate tracking issue since its very low risk to release and manually verify downstream (if it cannot be manually verified here). It may be possible by inspecting sess.Handlers similar to how the AWS Go SDK does unit testing in its repository.