feat(rpc): expose reqwest client

[KaiserKarel]

Previously, the reqwest client for HttpClient would be unconditionally built by the internal builder, meaning that useful middleware such as tower could not be applied.

This commits adds two ways to create the HttpClient with a custom reqwest::Client, either through new_from_parts, or using the builder.

• [ ] Referenced an issue explaining the need for the change -> this is a trivial code change.
• [x] Updated all relevant documentation in docs
• [x] Updated all code comments where relevant
• [ ] Wrote tests
• [x] Added entry in .changelog/

[romac]

@KaiserKarel Looks great, thanks for the PR! Can you please add a changelog entry for this, mentioning the new Builder::client and HttpClient::new_from_parts methods?

[romac]

Don't worry about the failure on nightly, I'll take care of it.