program: allow onramp in deposit/withdraw

[2501babe]

part 1/3 of #387. update DepositStake and WithdrawStake to gracefully handle the onramp account in the accounts list. once this is live, part 2/3 we add it to instruction builders. after communicating with any ecosystem teams needed, part 3/3 we make it mandatory in the program

this will enable us to change the token math to include onramp stake (or possibly all sol, will have to debate), which will further allow us to implement DepositSol

i know we typically stick new accounts at the end, but this will eventually be a hard breaking change, so i preferred to keep the "all instructions have all accounts in the same order" property because it pleases me. if we really need to avoid a breaking change, we could put a onramp_stake and last_replenished_epoch on the pool struct. i would rather avoid this however because, among other reasons, it adds complexity for no benefit

also i moved the mint errors up to match the account ordering

[2501babe]

my thinking is to do a release and bpf deployment after this pr, #390 before or after doesnt matter, and then update the instruction builders, probably update everything to sdk 3.0, and do another release (which doesnt require a bpf deployment)

[joncinque]

Looks great!