interface: Add cross-program invocation helpers

[febo]

Problem

Currently the interface crate does not offer a CPI helper.

Solution

Add CPI helpers following the pattern used in pinocchio. This includes a refactor of the interface to minimize dependencies. The crate now contains 2 features:

• "cpi": intended for on-chain program and enables CPI helpers
• "instruction": intended for off-chain clients and enables instruction helpers

[febo]

@joncinque @grod220 This is one alternative to refactor interface crates + add helpers. It is probably not the best example since it does not contain "state" types, but it could give an idea.

[joncinque]

Thanks for opening this up!

[joncinque]

instruction as a feature might encourage the wrong behavior, since the cpi helpers also create instructions (in a way).

I can get behind instruction if it's preferred by others, but we could go with alloc to be technically correct, since that's what solana-instruction requires, or we can go with offchain to make it clearer where it should be used.

Of the options, I think I prefer alloc the most.

[febo]

Yeah, the name is definitely not ideal, but I found it tricky to come up with a feature name for this. Maybe alloc would be best, although it does not directly reflect what is being enabled; offchain is tricky since anyone still using AccountInfo will need to use this feature to get an instruction for CPI.