lending: Add slippage constraint for borrow instruction

[jstarry]

Problem

When processing a borrow instruction which is denoted by deposited collateral, there is currently no way to specify the minimum number of borrowed tokens to receive.

Processing borrows denoted by requested borrow tokens does not need an explicit slippage constraint because collateral tokens can be restricted by using SPL token's delegate instruction. But it seems nice to have a slippage constraint for this direction too.

Proposed Changes

Add minimum amount to borrow instruction params

[jmalcic]

@joncinque would it be useful for me to work on this?

[joncinque]

@jmalcic certainly, this should be a fun one -- go for it!