(Ext) Add custom HD derivation path option for Trezor

The custom derivation path was merged for Ledger https://github.com/MetaMask/metamask-extension/pull/9367 (it is not yet available as of version 9.5.4, though).

Something similar might be needed for Trezor's integration too:

> @vondra: I moved from Ledger to Trezor. I need to connect to m/44'/60'/0' addresses. When I connect Trezor is there a option how to manualy set derivation path or invoke the path selector ?

[Comment](https://github.com/MetaMask/metamask-extension/issues/6716#issuecomment-836727593)

[This support article](https://metamask.zendesk.com/hc/en-us/articles/360060331752) will need to be updated once the Custom Derivation Path is available for either Ledger or Trezor.

Start by comparing the Ledger custom derivation path implementation in pull request #9367 with the existing Trezor integration. Confirm the requested m/44'/60'/0' path behavior and review the linked support article. Done means Trezor offers the equivalent custom path option and the support article reflects its availability.

Written by the indexing model from the issue text.