Permit pipenv and other tools to report itself as the downloader in pip's user agent

[chrahunt]

What's the problem this feature will solve?

Pip wrappers (like pipenv) want to be able to measure adoption of their tools.

Describe the solution you'd like

As discussed in #5424:

• submit a PR to update linehaul's UserAgent schema to include an extra optional wrapper field (tracked by Support wrapper field in Installer data parsed from User Agent pypi/linehaul#50)
• wait for it to be deployed
• update the JSON data in pip's UA to include the extra subfield with the wrapper info based on the PIP_USER_AGENT_WRAPPER_NAME and PIP_USER_AGENT_WRAPPER_VERSION environment variables

Alternative Solutions

• Allow setting arbitrary string in user-agent - could break existing format expected by linehaul
• Do nothing - not justified based on the presence of a valid use case which doesn't have available workarounds or is able to be handled by external tools

Additional context

• pypa/packaging-problems#58 (comment)
• Permit pipenv and other tools to report itself as the downloader in pip's user agent #5424
• Support wrapper field in Installer data parsed from User Agent pypi/linehaul#50