Support passing User-Agent

[radeksimko]

Tools executing Terraform (such as Terraform Language Server) would benefit from measuring impact/usage and one way of doing it would be by appending an identifier to User-Agent by passing TF_APPEND_USER_AGENT which then enables filtering data from whatever upstream backend requests were sent to.

https://github.com/hashicorp/terraform-plugin-test/issues/22

I'm not sure if this should be a first-class config option, but I'm creating this issue to discuss whether it should.

[paultyng]

I think this should be a first class option. We should probably just add a default value to the User-Agent with terraform-exec/v1.2.3 and append additionally to that if any value passed in the env var or manually.

This would result in 3 sources for the env var when passed down to the exec.Cmd:

1. constant value of terraform-exec
2. code supplied value via UserAgent option function
3. env variable already specified in os.Environ (similar to checkpoint handling somewhat)

All 3 should be appended when passed down to the CLI.

[paultyng]

May need to use #22 to get the version info.