Remove mutability of self in Exchange get methods

[mankinskin]

75

[mohammadowaisssss]

Checks cleared.

[ghost]

I would prefere using non mut version of mut but this will prevent nash from using openlimits. Nash uses a pool of MPC keys that needs to be mutated when doing some signatures.

@Ejhfast that would be good to opensource the nash rust sdk so people start seeing why we need to use mut self.