Add cash withdrawal API

**Is your feature request related to a problem? Please describe.**
Businesses may use Alpaca as their brokerage to invest idle cash. However, to deploy that cash (e.g. to pay employees a regular salary), the cash needs to be withdrawn from Alpaca periodically. There should be an API so that this can be automated.

**Describe the solution you'd like**
I guess there should be two APIs:
1. Fetch linked accounts.
2. Request a withdrawal from Alpaca to a linked account.

**Additional context**
Note that the above solution does not provide an API to link new accounts; it only allows transferring money to existing linked accounts. This avoids adding extra fraud risk.