Add httpx.Version configuration.

[tomchristie]

Discussion: #3026 (reply in thread)

I don't think we're clear on this yet, but drafting up as a proposal.

TODO:

• Docs.
• CHANGELOG.md.
• Ensure we have a clear section where we outline 1.0 version changes in one place.

[karpetrosyan]

I think we are going too far with encapsulating arguments :d

[tomchristie]

Valid perspective yes.

[T-256]

We should also have long term perspective since we are going to v1.0 release which then we should be semver compatible.
We may add http3 support and I think it's good to have one argument to managing client's http versions instead of three arguments.