rpc: add walletconnect support

[khoslaventures]

• rpc: add cosmos namespace for wallet connect
• rpc: add SignDirect

We are supporting: https://docs.walletconnect.org/v/2.0/json-rpc/cosmos

Description

Testing can be done by:

curl -X POST --data '{"jsonrpc":"2.0","method":"cosmos_getAccounts","params":[],"id":1}' -H "Content-Type: application/json" http://localhost:8545
curl -X POST --data '{"jsonrpc":"2.0","method":"cosmos_signDirect","params":[{"signerAddress":"eth1gqkt20afxpgaefp2cmy3w802th667j0cap6z98","signDoc":{"chainId":"ethermint-2","accountNumber":"1","authInfoBytes":"0a0a0a0012040a020801180112130a0d0a0575636f736d12043230303010c09a0c","bodyBytes":"0a90010a1c2f636f736d6f732e62616e6b2e763162657461312e4d736753656e6412700a2d636f736d6f7331706b707472653766646b6c366766727a6c65736a6a766878686c63337234676d6d6b38727336122d636f736d6f7331717970717870713971637273737a673270767871367273307a716733797963356c7a763778751a100a0575636f736d120731323334353637"}}],"id":1}' -H "Content-Type: application/json" http://localhost:8545

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[fedekunze]

@pedrouid for wallet connect v2 support

[khoslaventures]

closed in favor of #386