Add SwapperCalleePsm

[telome]

Some open questions:

• do we want to automatically fill() the PSM buffer when there is not enough dai available to be bought? Currently leaning towards no as large swaps by facilitators can call fill() manually and small swaps via StableSwapper may assume that keepers are regularly filling the PSM buffer. Swaps via StableSwapper may also be less common than for the UniV3 callee given that there is no slippage risk to mitigate here.

• do we want to use the data of the swapCallback() to pass the PSM address (and therefore support multiple PSMs more easily)? Currently leaning towards no as only the USDC PSM is currently considered and passing the PSM address as data implies additional swap costs.

• do we need a token recovery method in case the wrong gem is sent to the callee? Probably not necessary as only swappers are going to be warded callers of swapCallback().

[sunbreak1211]

do we want to automatically fill() the PSM buffer when there is not enough dai available to be bought?...
I'd keep it simple as it is.

do we want to use the data of the swapCallback() to pass the PSM address (and therefore support multiple PSMs more easily)?
I like as it is.

do we need a token recovery method in case the wrong gem is sent to the callee? Probably not necessary as only swappers are going to be warded callers of swapCallback().
Yeah, if following the regular path, this should not happen, as otherwise it would revert. We can't ofc prevent that anyone is sending tokens, but I think we shouldn't prevent that.

LGTM.

[oldchili]

LGTM