lang: Add #[instruction] attribute proc-macro

[acheroncrypto]

Problem

There is no way to override the default 8-byte Anchor discriminators of program instructions.

Summary of changes

Add #[instruction] attribute proc-macro that can be used to override Anchor defaults of program instructions.

This macro currently only allows overriding discriminators, but the functionality can be extended to override other defaults in the future.

Example

#[instruction(discriminator = [0, 1, 2, 3])]
pub fn my_ix(_ctx: Context<MyIx>) -> Result<()> {
    Ok(())
}

More examples can be found in the custom-discriminator tests.

Note

The initial design was to introduce a new #[discriminator] macro to share the implementation of instructions, accounts and events, as mentioned in #2728 (comment). However, given there is a demand from the users to override the other defaults of Anchor (e.g. serialization), it became clear that introducing a new macro for all of the overrides would not scale well.

---

Note: This PR is part of a greater effort explained in #3097.

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