RpcClient::send<T> now supports client-defined RPC methods via RpcRequest::Custom (backport #18881)

[mergify]

This is an automatic backport of pull request #18881 done by Mergify.
Cherry-pick of f264511 has failed:

On branch mergify/bp/v1.7/pr-18881
Your branch is up to date with 'origin/v1.7'.

You are currently cherry-picking commit f26451158.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   client/src/rpc_client.rs
	modified:   client/src/rpc_request.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   client/src/mock_sender.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/

[mergify]

automerge label removed due to a CI failure

[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #18882 (57ab243) into v1.7 (548ddff) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##             v1.7   #18882   +/-   ##
=======================================
  Coverage    82.3%    82.3%           
=======================================
  Files         434      434           
  Lines      122695   122714   +19     
=======================================
+ Hits       101008   101026   +18     
- Misses      21687    21688    +1