Allow the Bridge pool fees to be payed in more token types

[sug0]

Allow the Ethereum Bridge pool fees to be payed in more token types. This feature depends on #1693.

Associated tasks:

• Change the Bridge pool VP logic to accept more gas token types, and check their escrowed amounts
• Determine how to remove from the appropriate Bridge pool escrow token address (we're currently hardcoded to NAM)
• Change CLI arguments
• Fix build_bridge_pool_tx (must use fee token addr obtained from args)
• Test paying gas fees in NUTs is not possible
• Test paying gas fees with wrapped NAM (NUTs or ERC20s) is not possible
• Add tests checking fee token types other than NAM in the Bridge pool

---

Relevant CLI arguments:

      --fee-amount <fee-amount>
          The amount of NAM you wish to pay to have this transfer relayed to Ethereum.
      --fee-payer <fee-payer>
          The Namada address of the account paying the fee.
      --gas-amount <gas-amount>
          The amount being paid for the inclusion of this transaction
      --gas-token <gas-token>
          The token for paying the gas

[sug0]

Should we allow the Bridge pool fees to be payed in more token types, other than NAM?
cc @batconjurer

EDIT: Updated the issue's body. Originally it was just a question.

[batconjurer]

Yes, we should