Support custom discriminators

[acheroncrypto]

Problem

There are several problems regarding Anchor discriminators:

• Due to the transaction size limits enforced by the Solana runtime (1232 bytes), 8 bytes can be too high for some use cases (e.g. Allow Optionally Reducing The Instruction Sighash Length Or Matched Bytes #2006)
• The Discriminator trait has a fixed size type field ([u8; 8]) which means we can't implement it for non-Anchor programs (e.g. in declare_program!)
• Discriminators are not customizable
• Changing the name of the data type the discriminator was derived from results in a different discriminator

Solution

Don't make assumptions about the discriminator size and allow users to override the default Anchor discriminator. To do so, we need to complete the following tasks:

• Make discriminator type unsized (lang: Make discriminator type unsized #3098)
• Get discriminator length dynamically rather than hardcoding as 8 (lang: Get discriminator length dynamically #3101)
• Add non-8-byte discriminator support in declare_program! (lang: Add non-8-byte discriminator support in declare_program! #3103)
• Update dispatch function to support dynamic discriminators (lang: Update dispatch function to support dynamic discriminators #3104)
• Remove the fallback function shortcut in try_entry function (lang: Remove the fallback function shortcut in try_entry function #3109)
• Require Discriminator trait impl when using the zero constraint (lang: Require Discriminator trait impl when using the zero constraint #3118)
• Get discriminator lengths dynamically in TS client (ts: Get discriminator lengths dynamically #3120)
• Parse non-8-byte event discriminators in Rust client (client: Support non-8-byte discriminators #3125)
• Support overriding discriminators of:
  • Instructions (lang: Add #[instruction] attribute proc-macro #3137)
  • Accounts
    • Parse arguments properly (lang: Parse #[account] attribute arguments with syn #3140)
    • Use the associated discriminator constant (lang: Use associated discriminator constants instead of hardcoding in #[account] #3144)
    • Fix compilation with generics (lang: Fix compilation error when an #[account] struct has generics #3148)
    • Add discriminator argument (lang: Add discriminator argument to #[account] attribute #3149)
  • Events (lang: Add discriminator argument to #[event] attribute #3152)
• Reuse common override arguments (lang: Reuse common override arguments #3154)
• Check potential discriminator collisions (idl: Check ambiguous discriminators #3157)
• Disallow all zero account discriminators (idl: Disallow all zero account discriminators #3159)
• Update discriminator errors (lang, ts: Remove "8 byte" requirement from discriminator error messages #3161)
• Remove Discriminator::discriminator (lang: Remove discriminator method from Discriminator trait #3163)
• Support non-8-byte discriminators in CLI (cli: Support non-8-byte discriminators #3165)
• Disallow empty discriminators (idl: Disallow empty discriminators #3166)
• Refactor discriminator generation (lang: Refactor discriminator generation #3182)
• Deprecate the #[instruction] attribute (lang: Deprecate #[interface] attribute #3195)
• Update discriminator-related documentation (lang, ts: Update discriminator-related documentation #3211)