refactor(boost): add ServiceHandle future

[mattsse]

• rename async fn Service::run to fn Service::spawn() -> ServiceHandle

• add ServiceHandle Future that waits for both task to finish

since this launches the server this could be improved to wait until the server is up, but can tackle that after looking at BlindedBlockProviderServer, so perhaps this PR is premature

[mattsse]

split Server::run into two functions

• serve: constructs the axum server
• spawn: constructs the axum server and spawns to new task

this has the benefit that after calling serve the server is already bound and listens for incoming connections

the ServiceHandle is currently duplicated. imo this fine

[ralexstokes]

sorry for the delay on this! I haven't used pin_project before so wanted to be able to give it some attention before merging it in

this is excellent and thanks for the contribution :)

this change is strictly better imo as the API now gives better control over when and how the various tasks are constructed/orchestrated -- we can see this where you drop the arbitrary sleep from the integration test code

[ralexstokes]

thanks for this! I like adding the ServiceHandle abstraction to provide better granularity around orchestrating the various tasks as they boot and then run