feat: add minimal framework for spec test

[ensi321]

How to run

zig build run:download_spec_tests && zig run:write_spec_tests && zig test:spec_tests

Sample result

Part of #21

[twoeths]

Suggested change

	try ST.deserializeFromBytes(serialized, value);
	try ST.deserializeFromBytes(serialized, &@field(out, fld.name));

hence no need to declare value here
we can simplify "*Out" not to contain pointers, for example

pub const Phase0OperationsOut = struct {
    pre: ?ssz.phase0.BeaconState.Type = null,
    post: ?ssz.phase0.BeaconState.Type = null,
    attestation: ?ssz.phase0.Attestation.Type = null,
    attester_slashing: ?ssz.phase0.AttesterSlashing.Type = null,
    block: ?ssz.phase0.BeaconBlock.Type = null,
    deposit: ?ssz.phase0.Deposit.Type = null,
    proposer_slashing: ?ssz.phase0.ProposerSlashing.Type = null,
    voluntary_exit: ?ssz.phase0.SignedVoluntaryExit.Type = null,
};

and we don't need an inline for to deinit fields at consumer side https://github.com/ChainSafe/state-transition-z/pull/46/files#diff-1d6f5b6eb82151a80442a6c728e68021970e2170f6ab4bb3574ac4f44b10886fR27

[ensi321]

@twoeths The thing is each field has default value of null. So &@field(out, fld.name) is a pointer to a nullable, which ST.deserializeFromBytes doesn't accept.

We need to pass a ST.Type pointer to ST.deserializeFromBytes by creating one, and then assign the field to it.

[twoeths]

general approach looks good to me, left some suggestions to deduplicate schemas and simplify things a bit, let me know if it does not work