bridge: implement keygen command

[leoluk]

Stack from ghstack:

• bridge/pkg/ethereum: remove channel unsubscribes #109 bridge/pkg/ethereum: remove channel unsubscribes
• bridge: implement guardian set update submission node admin service #104 bridge: implement guardian set update submission node admin service
• terra: disable in production mode #103 terra: disable in production mode
• bridge: refactor out broadcastSignature to prepare for injection path #102 bridge: refactor out broadcastSignature to prepare for injection path
• solana: verify that new guardian set isn't empty #101 solana: verify that new guardian set isn't empty
• docs: add simple overview image #92 docs: add simple overview image
• bridge: implement keygen command #91 bridge: implement keygen command
• bridge: implement bridge key serialization #90 bridge: implement bridge key serialization
• ethereum: update packages and use package-lock.json #89 ethereum: update packages and use package-lock.json

Tested using /guardiand keygen /bar --desc foobar.

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This change is