`check_limits_within`: clarify meaning of amount arg

[ok300]

Before, limits.within() and check_limits_within() checked if the output_amount was within the limits min / max.

However, interacting with the Boltz Web UI shows that the limits validation actually applies to the input amount, i.e. the amount that will be swapped.

This PR renames the output_amount to swap_amount for clarity.

[ok300]

I checked all swap types in the Boltz UI and it's a bit more complicated:

Swap Type	Checked to fit within pair limits	Notes
LN -> BTC	input	invoice amount
BTC -> LN	output	invoice amount
LN -> LBTC	input	invoice amount
LBTC -> LN	output	invoice amount
BTC <-> LBTC	input

I'll mark this as a draft until I figure out a simple way to cover all these cases.

[i5hi]

Hey @ok300 Please setup commit signing.