Add a Route function on 02-client's Keeper

[DimitrisJim]

See discussions #5806 (comment) and #5806 (comment).

Can add an additional method on the keeper to directly grab the light client module for a given client id. GetRouter can be kept for cases where we want to directly manipulate the router mapping, most light client code shouldn't directly manipulate that mapping, only access its contents.

Adding needs discussion label just in case, even though there appears to be consensus behind adding it.

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[DimitrisJim]

opened PR for this, as a draft for now. One singular comment to remove the needs discussion will do to move it to open