Send better user-agent values

[chris48s]

📋 Description

At the moment we send a hard-coded user-agent header.

It would be nice if:

• Shields.io itself sent a different value than self-hosted instances send by default (obviously customisation/spoofing is possible)
• The user agent contained some kind of meaningful indication of the server version

One useful thing we can do is write the version to a file we can read from when we prep a release.
We would probably also want to set a default user agent and then make it configurable via an env var we can set in production.

A bit more discussion on this: #6256 (comment)