Configuration support fully specified source to destination mapping

[michaelkaplan13]

Context and scope
Currently, the relayer configuration contains "source subnets" and "destination subnets", such that the relayer will try to relay all messages from any of the source subnets sent to any of the destination subnets. It will likely be desirable to be able specify the specific point-to-point connections that the relayer should support. This would apply to the following use case of a new subnet being created (call it Subnet XYZ), and the project wanting to provide interoperability with two other existing subnets (call them Subnet A and Subnet B). In this case, they would want to set up a relayer that supports XYZ <-> A, and XYZ <-> B, but not A <-> B

This could be implemented by adding additional configuration options that specify the full definition of the mapping for messages to be relayed, rather than assuming an "all sources to all destinations" mapping.