Introduce datagram domain type

[adizere]

Summary of Bug

We currently use the term "message" to refer to the various data types that IBC handlers consume. We should consider introducing "datagram" to replace "messages", as a more accurate term (also reflecting the ICS specs more closely).

Note that this issue depends on coordinating with external teams, namely we should probably get input from the spec team (cosmos/ics) and SDK team to ensure that the "datagram" vs. "message" disambiguation is consistent across teams.

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[adizere]

Another improvement we can throw into the same bucket with datagrams is the naming practice around our serialization types:

Looking at the code now that this new Protobuf type is kicking in, I'm wondering if we should not adapt our practice of calling serialization types with the "Raw" prefix. Instead, we could use the "Proto" prefix or something similar. So for example we'd have the following:

impl Protobuf<ProtoClientConnections> for ConnectionIds {}

The idea is to make the connection between tendermint_proto::Protobuf and the underlying type more explicit. It's mostly aesthetics, but might make the codebase simpler to read & understand.

Originally posted by @adizere in #364 (comment)

[adizere]

Closing as outdated. The "datagram" term never took off as part of the IBC ecosystem (SDK/ibc-go/go-rly/etc).

The suggestion in this comment is a nice to have, but unclear what/if it solves any issue.