json-rpc-engine: Replace use of any with proper types (non-test files only)

[mcmire]

Look for the following string in non-test files to find these: // TODO: Replace `any` with type

At time of writing, there were 6 instances.

[MajorLift]

• Completing this ticket will enable us to take full advantage of the type safety improvements that will be introduced by the provider alignment technical proposals.
• All of the any usages in this package are for typing errors in catch blocks. These errors should mostly be aligned to JsonRpcEngineCallbackError.
• serializeError() should only be used when sending an error via JSON-RPC. Otherwise, the information in unknown errors thrown by middleware should be preserved.

[MajorLift]

• See previous refactor efforts: https://github.com/MetaMask/json-rpc-engine/pulls?q=is%3Apr+is%3Aopen+sort%3Aupdated-desc
• Especially relevant: WIP: Improve middleware errors json-rpc-engine#84