feat: add a way to provide a custom User-Agent

[freak12techno]

Fixes https://github.com/informalsystems/tendermint-rs/issues/1425.

Need this to be able to specify a custom User-Agent for Hermes requests.

I am unsure how to test it properly with Hermes though; if someone can give me any hint on how to compile Hermes with this version of tendermint-rpc, I'd appreciate that.

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

[freak12techno]

@romac can you review this or ask someone to do so? also, if you know how to compile Hermes with this version of tendermint-rpc, this would help testing using custom User-Agent on Hermes itself. disregard that, I figured it out.