feat(lightpush): validate messages before relaying

[jm-clius]

Problem

Lightpush service nodes will currently attempt to publish/relay messages received from clients without checking the message validity first.
Messages may violate violation rules for a variety of different reasons, including exceeding the maximum size, having an invalid RLN proof, etc. See the network spec for more.
This prevents the lightpush service node from returning an appropriate error message to the client.
At least two reported bugs will be fixed with message validation checking in the lightpush service node:

• bug(rln-relay): light push does not check proofs before forwarding to relay #1226
• bug: strange errors when light pushing messages with payload >= 300 kb #2565

Validation checking in the Relay REST API can be used as inspiration:

nwaku/waku/waku_api/rest/relay/handlers.nim

Lines 155 to 156 in 006d43a

	(await node.wakuRelay.validateMessage(pubsubTopic, message)).isOkOr:
	return RestApiResponse.badRequest("Failed to publish: " & error)

More resources: lightpush spec

Acceptance criteria

• lightpush service node checks message validity before publishing
• lightpush service node returns useful error to clients on validation failure
• unit tests for most common message validation failure scenarios

[fryorcraken]

Considering "Provision RLN for light push clients PoC" milestone, this does not seem needed at this point in time.

[jm-clius]

Considering "Provision RLN for light push clients PoC" milestone, this does not seem needed at this point in time.

This will be the first step for a RLN proofs service (validating the message - with provisioned proof - before publishing). In fact, I think this issue can be closed with the merging of #2695. @shash256 please confirm and close if addressed.

[shash256]

Yes, I am closing this issue now as we have validation in light push already merged.