Limit max size of payloads

[gjermundgaraba]

To avoid DDOS scenarios with IBC, we should limit the maximum size of packets by limiting the payload size. We should do this for both v1 and v2 packets.

We will set the max limit to 262144 bytes.

For v1, we can check the data size in modules/core/04-channel/keeper/packet.go: SendPacket and RecvPacket.

For v2, we can check the total packets total payload size (sum packet.payloads sizes) in modules/core/04-channel/v2/keeper/msg_server.go: SendPacket and RecvPacket.

[DimitrisJim]

Ref relevant spec issue cosmos/ibc#1160

(though spec would in theory only mention implementations could opt to add limits, so hashing out what these look like is ibc-go level discussion)