Ability for user to insert provisional User-Agent string

[conspicuousClockwork]

What's the problem this feature will solve?
Some end users have a specific need to modify the User-Agent header. As per issue #5424, direct overwriting is a drastic measure, so instead, this is a request to add provisional data to the JSON appended to the header by default.

Describe the solution you'd like
Usage of the environment variable PIP_USER_AGENT_PROVISIONAL_STRING to insert data into the JSON appended onto the default User-Agent header.

This will allow users who are required to include specific information in the User-Agent headers sent out (such as through a proxy) and will allow users to implement their own tracking on Pip requests as well.

Alternative Solutions
Currently the only alternative solution is to modify download.py by hand to include a custom value to the User-Agent header.

Additional context
This is similar to #5424, however, that issue veered towards support for wrappers. This issue is focused on providing this functionality to end users while allowing for the eventual support of wrapper specific variables.

[pfmoore]

Currently the only alternative solution is to modify download.py by hand to include a custom value to the User-Agent header.

One other alternative solution (I'm not saying it's necessarily a good one, just mentioning it as an option) would be to run a local proxy on your PC that injected the extra header information while forwarding the traffic to the original destination.

[conspicuousClockwork]

@pfmoore true, although several other package management utilities include this functionality so there seems to be a precedent for having it. I'm about to create a pull request for this, and it didn't require too much modification to be honest.

[conspicuousClockwork]

@pfmoore any more thoughts on this? The original discussion surrounding the user agent header was intended for wrappers, so I'm not entirely sure if the suggestion of a complete override was scrapped because it was for the purpose of a wrapper or not.

A few other utilities allow for the complete overriding of the user-agent string, but if there are concerns about that it is completely understandable. I'm just trying to get this functionality for my team, since we are all affected by a lack of a way to authenticate properly through our proxy. Thanks!

[pfmoore]

Not from me, no.

[conspicuousClockwork]

Ah, okay. I'll wait for other members to review this issue and PR then. Thanks for the input!

[cjerdonek]

Addressed by PR #5550 / ac9010e.

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