Add support for gateway deposits/withdrawals

[grctest]

There's gateway services built on top of the Bitshares DEX enabling you to deposit external chain funds in return for on-chain EBA tokens.

Currently in order to deposit/withdraw these funds you need to use either the reference web wallet or the gateway's own reference web wallet.

If we add support for these gateways to the Astro UI then the user won't need to navigate to another app/service to deposit/withdraw funds.

[grctest]

Out of scope for now - will direct users to the gateway's web wallets so they use their system.