Support for decryption functionality like in MetaMask Extension

#Description
As a developer, I want to be able to use the new methods `eth_getEncryptionPublicKey` and `eth_decrypt` ([docs](https://docs.metamask.io/guide/experimental-apis.html#eth-rpc-methods)), that were added to the MetaMask Browser Extension [in v8.0.0](https://github.com/MetaMask/metamask-extension/issues/1190#issuecomment-643487002), in the MetaMask mobile app too, so that my users have a consistent experience using my web app on desktop and on mobile.

**Acceptance Criteria**
- Support for `eth_getEncryptionPublicKey` as well as `eth_decrypt`

No files, tests, or entry points are named. Start by reading the linked MetaMask experimental API documentation and locating the mobile app's handling for Ethereum provider RPC methods. Done means the mobile app supports both eth_getEncryptionPublicKey and eth_decrypt consistently with the stated acceptance criteria.

Written by the indexing model from the issue text.