rpc: refactor rpc packages and backend to support cosmos namespace

[fedekunze]

Description

This PR refactors the RPC package so that it's easier to implement the "cosmos" namespace per Wallet Connect V2. See Changelog for a detailed list of API breaking changes

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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)

[semgrep-app]

Semgrep found 1 hidden-goroutine finding:

• rpc/namespaces/ethereum/eth/filters/subscription.go: L35-52

Detected a hidden goroutine. Function invocations are expected to synchronous, and this function will execute asynchronously because all it does is call a goroutine. Instead, remove the internal goroutine and call the function using 'go'.

_{⚪️ This finding does not block your pull request.
🙈 From go.lang.best-practice.hidden-goroutine.hidden-goroutine.}

[loredanacirstea]

utACK
I understand that this PR prepares the cosmos namespace to be added on the same level as the Ethereum namespaces.

[crypto-facs]

LGTM