lang: Reuse common override arguments

[acheroncrypto]

Problem

The new discriminator = <EXPR> argument can be used with #[instruction], #[account], and #[event] attributes, but they don't share code between each other, which results in unnecessary duplication.

Summary of changes

Add a new Overrides struct that can be shared between all previously mentioned attributes above.

The implementation is slightly different for #[account] because it also has other arguments (zero copy and namespaces) that we'd like to keep backwards-compatible.

This change achieves the same functionality with less than half of the previous code.

---

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

[vercel]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.