idl: Add separate spec crate

[acheroncrypto]

Problem

Making changes to the IDL crate, e.g. adding features such as the convert feature, would require bumping the version even if the spec itself doesn't change. We essentially have two options to fix this:

• Remove the patch version from the spec field i.e. use 0.1 instead of 0.1.0 so that patch changes to the IDL crate version does not affect the spec field
• Add a new spec crate that only contains the IDL type specification so that the crate only gets a new version when there is an actual spec change

While both options seems viable, combined with the reasons explained in https://github.com/coral-xyz/anchor/pull/2901, the latter option seems to be more suitable.

Summary of changes

Add a separate anchor-lang-idl-spec crate that only contains the IDL type specification, and re-export it from the main IDL crate to keep backwards compatibility.

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