Allow to start Substrate service without RPC server

[nazar-pc]

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Motivation

We use Substrate as a library, meaning we don't use CLI interface, have custom logger setup, etc.

One of the things that we don't always need is RPC server (for example in a desktop app where we can retrieve all necessary information programmatically instead of via RPC), but right now it is started unconditionally.

Request

Allow to configure service with RPC server not enabled at all. It is already possible with custom spawn_tasks implementation, but requires copy-pasting a bunch of code and nicer API would be appreciated.

Solution

With #5364 configuration now has rpc: RpcConfiguration field, maybe as simply as changing it to rpc: Option<RpcConfiguration>.

cc @niklasad1

Are you willing to help with this request?

Maybe (please elaborate above)