Add the possibility to override User-Agent for queries

[freak12techno]

Description

Required for informalsystems/hermes#3979.

TLDR: there should be a way for the library users', including Hermes, to override the User-Agent header for all the queries such an app does, so it would be easier for the node owners to understand which app is doing which queries, given that the app overrides User-Agent using this feature.

Definition of "done"

A library provides a way to specify a custom User-Agent when creating a HttpClient.

[freak12techno]

@romac can we bump tendermint-rs to be able to use the new version in hermes?

[romac]

I will release this by end of week and then we can bump tendermint-rs in Hermes and set an appropriate user agent.

[freak12techno]

@romac lovely. Can you ping me once it's done?
Also, feel free to assign me to informalsystems/hermes#3979, I basically have already done it, except for a new things.