Add support for custom client ids

[srdtrk]

Summary

Right now, every client has a universal client id such as client-0. This proposes letting an authority use custom client ids such as cosmoshub. Similar to PORT_CUSTOMIZER_ROLE in ICS26Router.sol.

Permissionless users would still be able to register their light clients using the universal client ids.

Constraints

The custom client ids must not be allowed to be prefixed with client- or channel-. They must go through the same string validation as port ids.

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