Make launch_addons easier to consume

[mattsse]

Describe the feature

reth/crates/node/builder/src/rpc.rs

Line 413 in 4a0bc37

async fn launch_add_ons(self, ctx: AddOnsContext<'_, N>) -> eyre::Result<Self::Handle> {

currently op and eth both use the same code:

reth/crates/optimism/node/src/node.rs

Line 167 in 4a0bc37

}

we'd like to modify this step entirely, hence we need to extract this to a helper type that provides reusable functions, like spawning the servers etc.

Additional context

No response