Rpc pubkey

[mratsim]

This centralizes all crypto handling in crypto.nim instead of having it sprawled out.

In particular, for REST and RPC, pubkeys are supplied by users and due to lazy validation, those are unchecked (no check that they are infnity, on curve or in subgroup).
Fortunately:

1. We require the endpoint to be exposed only to trusted services (either local or in a safe network)
2. All codepaths in REST and RPC compares the pubkey with the known one from the Ethereum Beacon State, hence invalid keys cannot slip in.

Alternative: since RPC is scheduled for deprecation, we can also deleted the related files instead of merging this PR.

Regarding REST, validators pubkeys go through ValidatorIdent and/or node.restKeysCache and those are compared to the known validators key before use so no issue there:

nimbus-eth2/beacon_chain/rpc/rest_beacon_api.nim

Lines 339 to 362 in 13b264d

	let vindex =
	block:
	let vid = validator_id.get()
	case vid.kind
	of ValidatorQueryKind.Key:
	let optIndices = keysToIndices(node.restKeysCache, stateData.data,
	[vid.key])
	if optIndices[0].isNone():
	return RestApiResponse.jsonError(Http404, ValidatorNotFoundError)
	optIndices[0].get()
	of ValidatorQueryKind.Index:
	let vres = vid.index.toValidatorIndex()
	if vres.isErr():
	case vres.error()
	of ValidatorIndexError.TooHighValue:
	return RestApiResponse.jsonError(Http400,
	TooHighValidatorIndexValueError)
	of ValidatorIndexError.UnsupportedValue:
	return RestApiResponse.jsonError(Http500,
	UnsupportedValidatorIndexValueError)
	let index = vres.get()
	if uint64(index) >= validatorsCount:
	return RestApiResponse.jsonError(Http404, ValidatorNotFoundError)
	index

Closes #1715

[arnetheduck]

lgtm - debt of this kind has a tendency to spread, even for deprecated stuff

[github-actions]

Unit Test Results

     12 files  ±  0     829 suites  +8   26m 41s ⏱️ - 2m 30s
1 674 tests +  2  1 626 ✔️ +1    48 💤 +1  0 ❌ ±0 
9 775 runs  +16  9 663 ✔️ +8  112 💤 +8  0 ❌ ±0 

Results for commit 3351d3e. ± Comparison against base commit 13b264d.