Governance gated chains for bridging credits

[ryanchristo]

Summary

In #1101, we introduced a validation check for target in MsgBridge currently restricted to "polygon". In the future, allowed chains should be managed by governance, same as we do with allowed denoms and allowed credit class creators.

Problem Definition

In the future, we plan to support bridging assets to and from multiple chains. We need to validate the source and target chain to prevent any undesired behavior and the allowed chains should ultimately be decided through a governance process.

Proposal

• Add allowed chains to state using an ORM table
• Add transaction messages for adding and removing an allowed chain
• Add events for adding and removing an allowed chain
• Add a query for querying allowed chains
• Add corresponding CLI commands

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