feat: requests to remote always include user-agent

[lukevmorris]

I have a grpc Remote Execution API server that I use as a backend remote cache for both pants and bazel. I want to be able to distinguish between requests that came from bazel and those that came from pants. I originally relied on the user agent header but found that it was omitted if I configured any other headers.

In the reference sections for remote_execution_headers and remote_store_headers, the pants docs state that "Pants may add additional headers." I think User-Agent is one header that would be useful if always present.

This PR removes User-Agent from DEFAULT_EXECUTION_OPTIONS and opts instead to merge it with user-provided headers. That is, the following configuration:

[GLOBAL]
remote_store_headers = {"Authorization" = "Basic abc123"}

would result in the following HTTP Headers:

Authorization: Basic abc123
User-Agent: pants/2.21.0

These changes are intended to allow the user, if needed, to provide a custom user agent by setting it explicitly in remote_store_headers or remote_execution_headers. I'm not deeply familiar with python, so please let me know if there are any changes you'd like me to make. Thank you!