client: Support non-8-byte discriminators

[acheroncrypto]

Problem

anchor-client assumes discriminator length to always equal 8:

anchor/client/src/lib.rs

Lines 385 to 391 in 9fce3df

	let mut slice: &[u8] = &borsh_bytes[..];
	let disc: [u8; 8] = {
	let mut disc = [0; 8];
	disc.copy_from_slice(&borsh_bytes[..8]);
	slice = &slice[8..];
	disc
	};

Summary of changes

Get the discriminator length dynamically rather than hardcoding as 8.

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

[vercel]

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