Allow user to define derivation path

**What problem are you trying to solve?**
It seems there are a few cases where people would like to use derivation paths not currently supported by MetaMask.

- https://github.com/MetaMask/metamask-extension/issues/6716
- https://github.com/MetaMask/metamask-extension/issues/7238
- I myself would like to use the path `44'/1'/0'/0` for my Rinkeby accounts since this is also what [Frame](https://frame.sh) uses for Rinkeby accounts.

**Describe the solution you'd like**
I think rather than adding every possible derivation path users could want as a pre-set, MetaMask could simply give users the ability to set their own derivation path when setting up the wallet, for software and hardware wallets alike.

No files or tests are named. Start by reviewing the wallet setup flow and the related issues 6716 and 7238, then compare the software- and hardware-wallet cases described here. Done means users can define a derivation path during wallet setup, including the Rinkeby path cited in the issue.

Written by the indexing model from the issue text.