Implement `generate_debt`

[0x-r4bbit]

generate_debt(amount) in stablecoin/src/:

• Assert the position account is already initialized
• Accrue any outstanding stability fees on the position first
• Check that minting amount stablecoins keeps the collateralization ratio above the minimum (requires price feed and redemption price)
• Issue a chained Mint call to the token program to mint amount stablecoins to the user's stablecoin holding account
• Increase Position.debt_amount, initialize the per-position stability fee accumulator index

Also extend open_position with an optional debt_amount parameter: if non-zero, run the same collateralization check and mint after depositing collateral.

Depends on: feedback controller / redemption price, stability fees and price feed