Suggest custom RPC deeplink

### **Description**

I would like to create a deeplink which creates a suggestion to add a custom RPC to Metamask. The proper syntax to create such a link is not available in the documentation or via the deeplink generator. Could you please provide it?

### **References**
According to this issue, it should be possible:
https://github.com/MetaMask/metamask-mobile/issues/74

Chainlist.org uses the appropriate functionality with the "Add to MetaMask" buttons, but the underlying syntax isn't to be found.

No file or test is named. Start by reviewing the linked issue and the existing deeplink generator and documentation to identify the supported custom-RPC syntax. Done means the syntax is documented and, if the generator is the intended location, can produce the corresponding link.

Written by the indexing model from the issue text.