Extract the type_url explicit strings into constants

[ancazamfir]

Summary

With the migration to the new protobufs we need to keep track of the type_url used by prost_types::Any.
Currently these are used in clear, e.g.:
https://github.com/informalsystems/ibc-rs/blob/3e60e67b679ffab65b1a173b1af12411eb08d4c5/modules/src/ics02_client/client_def.rs#L128-L130

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[adizere]

We should use the fn type_url from trait Msg to encapsulate all calls to type_url:

https://github.com/informalsystems/ibc-rs/blob/12cb1d6187e878eee0e40295fe0b804ddef0b846/modules/src/tx_msg.rs#L21-L23

Currently this is only implemented for a handful of type, e.g., MsgCreateAnyClient:

https://github.com/informalsystems/ibc-rs/blob/12cb1d6187e878eee0e40295fe0b804ddef0b846/modules/src/ics02_client/msgs/create_client.rs#L81-L83

[ancazamfir]

This has been taken care over a number of PRs (see git blame :))