lang: Add discriminator argument to #[event] attribute

[acheroncrypto]

Problem

It's not possible to override the discriminator generated by the #[event] attribute.

Summary of changes

Add discriminator = <EXPR> argument to the #[event] attribute similar to #3137 and #3149.

Example

#[event(discriminator = 1)]
pub struct MyAccount {
    pub field: u8,
}

See a full example in custom-discriminator tests.

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

[vercel]

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