chore(rpc): trait for blocking task

[emhane]

Simplifies the interface for executing blocking code in EthApi.

• Removes semantically counterintuitive function for running a future on a blocking task Moves EthApi::on_blocking_task into trait method SpawnBlocking::spawn_blocking_io, which replaces less general EthTransactions::spawn_blocking. Moves EthApi::spawn_tracing_task_with into SpawnBlocking::spawn_tracing, replacing less general EthTransactions::spawn_blocking.
• Shrinks scope of code to execute on blocking task, to encompass only sync code, by moving async code into outer scope
• Moves methods for spawning blocking threads into trait, to access via default trait method impls in EthBlocks and EthTransactions
• Replaces calls to EthApi::on_blocking_task with calls to eth::api::SpawnBlocking::spawn_blocking_io in impl of EthApiServer. Moves closer to goal of eventually having default trait method impls for EthApiServer.

[emhane]

I don't get why this test doesn't work test_ensure_backwards_compatibility. it's unrelated to changes in this pr even, no? @mattsse

besides, it works locally :'(

[mattsse]

this change is problematic because it no longer treats CPU-bound and IO-bound tasks differently and spawns them in the pool of blocking tasks, which is a rayon wrapper intended only for tracing tasks that are predominantly CPU-bound

[mattsse]

lgtm, trait suggestion