[REQ] More fine-grained specification of http user agent

Is your feature request related to a problem? Please describe.

I would like my generated code to send header like <mycompany>/{{packageVersion}}/<language>. Right now, it is either OpenAPI-Generator/{{packageVersion}}/<language> or a provided --http-user-agent string from command line. This string, however, is not subject to further expansion, so I can't provide something like {{mycompany}}/{{packageVersion}}/<language> as a value. What I'd really like is being able to override just the first part while keeping /{{packageVersion}}/<language>.

Describe the solution you'd like

I think templating the first part of the default http-user-agent would actually be sufficient (and easy) solution. I guess it would be better to call the variable {{httpUserAgentIdentifier}} or {{httpUserAgentProduct}}, because these are the terms that RFC 7231 uses. This variable would default to OpenAPI-Generator (so the current hardcoded value) => nothing would change by default.

Describe alternatives you've considered

We could probably also make the value provided by --http-user-agent be a template, so user could provide something like FooBar/{{packageVersion}}/{{language}} on the command line, but I'd be afraid that this approach could open a can of worms as we'd have to start considering all variables a subject to expansion which would lead to more complex code and weird edge cases.

Additional context

None.

Start by tracing the --http-user-agent option and the existing default user-agent handling through the generator. Done means the default remains unchanged while the identifier or product prefix can be customized independently and packageVersion and language expansion still work.