feat: add esplora client timeout option

[Arowolokehinde]

Description

Closes #1109

EsploraClient::new had no way to set a request timeout: proxy was the
only setting it applied to esplora_client::Builder, leaving the
builder's timeout at None. A server that accepts a connection then stops
responding could block the calling thread indefinitely.

This adds an optional timeout (seconds), mirroring ElectrumClient::new. It
defaults to None, so callers opt in.

Notes to the reviewers

• Defaults to None as suggested, so nothing changes for existing callers.
  Added a docstring note for the hang risk when it's unset.
• Option<u8> seconds matches ElectrumClient.
• Verified against a local stub server that accepts a connection and never
  replies. With timeout unset the call never returned; with timeout = 3 it
  raised Minreq("the timeout of the request was reached") after 3.01s —
  catchable rather than a hang.
• No test added - this needs a server that accepts and then stalls.
• I don't have permissions to add labels - could someone apply
  changelog: added?

Documentation

• Other: esplora_client::Builder::timeout

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added docs for the new feature

[j-kon]

ACK 853a5ba