Support explicitly specified function kinds

[itegulov]

We have been distinguishing function kind purely by its usage of self which has been working out fine historically, but now that we have a (public) ABI that can be generated from a contract, we need a deterministic way of marking functions’ kinds (#[view], #[call] etc).

Whether the self-checks should still be in place is an open question, but I feel like SDK shouldn't care that I want to declare my foo(&self, a: u32) as a call function since there could be legitimate patterns where it makes sense (e.g. using a transaction-only env functions without touching the state).

CC @miraclx @austinabell as I have discussed this with you two independently