beacon/light/request: simple request framework

[zsfelfoldi]

This PR implements a general-purpose framework for sync mechanisms requesting data from multiple servers. It is intended to be used primarily by the beacon chain light client but might be used for other purposes too as it does nothing specific to the beacon chain. It implements timeouts, very simple rate limits and server selection, basically abstracting away the multiple server logic from the actual sync processes.

[holiman]

Doesn't compile?

?   	github.com/ethereum/go-ethereum/beacon/engine	[no test files]
ok  	github.com/ethereum/go-ethereum/beacon/light	0.185s
# github.com/ethereum/go-ethereum/beacon/light/request [github.com/ethereum/go-ethereum/beacon/light/request.test]
beacon\light\request\scheduler.go:220:46: syntax error: unexpected bool in argument list; possibly missing comma or )
FAIL	github.com/ethereum/go-ethereum/beacon/light/request [build failed]
?   	github.com/ethereum/go-ethereum/beacon/merkle	[no test files]
?   	github.com/ethereum/go-ethereum/beacon/params	[no test files]

[zsfelfoldi]

Doesn't compile?

Indeed, last commit had a missing bracket, fixed.

[fjl]

Included in #28822, which was merged.