[Infra] Add Shannon to the Cosmos chain registry

[Olshansk]

Objective

Add Pocket to the chain registry: https://github.com/cosmos/chain-registry

Origin Document

@studna mentioned that we're not on the Cosmos SDK chain registry and there's definitely value there.

Goals

• Increase the visibility of Shannon

Deliverables

• Add Shannon to the testnets directory

Non-goals / Non-deliverables

• Do anything else

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit and/or E2E) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @Olshansk
Co-Owners: @okdas