chore: store tokio::runtime::Handle in ethers/alloyDB

[chirag-bgh]

ref #1534

[DaniPopes]

can you also return None if the runtime flavor is CurrentThread, and can you document it briefly in the fn new doc comments saying that it requires a tokio runtime?

otherwise looks good

resolved

[wtdcode]

@rakita This change is not appropriate. If you wish to enforce ethersdb/alloydb to always play with async world, you should mark fn new as async fn new even not using any await in the function. This removes the check and also presents the correct signature.

@DaniPopes I disagree with not allowing ethersdb/alloydb to create current thread runtime. This makes them totally incompatible with sync world and creating a current thread runtime is rather cheap.