rpc: re-use server builder per rpc interface

[niklasad1]

This PR changes that the server builder is created once and shared/cloned for each connection to avoid some extra overhead to construct this for each connection (as it was before).

I don't know why I constructed a new builder for each connection because it's not needed but shouldn't make a big difference to my understanding.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/12033419110
Failed job name: fmt

[niklasad1]

bot fmt

[command-bot]

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[command-bot]

@niklasad1 Command "$PIPELINE_SCRIPTS_DIR/commands/fmt/fmt.sh" has finished. Result: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7820153 has finished. If any artifacts were generated, you can download them from https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7820153/artifacts/download.