Make Rust `bindgen` build dependency optional

[DaniPopes]

Now that header definitions are stable across builds (#377), we can remove the Rust bindgen build dependency by gating it to an optional feature. This also transitively removes the dependency on libclang for Rust bindings.

[DaniPopes]

Looks like it will suppress diffs for GitHub PRs

Yep, and excludes from statistics like the language percentages

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Just realized that this fails on Windows because the C_KZG_RET enum repr is i32 instead of u32: https://github.com/ethereum/c-kzg-4844/actions/runs/6839391407/job/18597486957?pr=382

I'd disable this for Windows because it's pretty minor, but let me know what I should do

[jtraglia]

Ah Windows... It would be nice if this worked for all platforms.

My first thought is to force the enum to be unsigned in C. It's not ideal, but this should work:

--- a/src/c_kzg_4844.h
+++ b/src/c_kzg_4844.h
@@ -94,9 +94,10 @@ typedef Bytes48 KZGProof;

 /**
  * The common return type for all routines in which something can go wrong.
+ * Note, values are large to force enum type to be an unsigned integer.
  */
 typedef enum {
-    C_KZG_OK = 0,  /**< Success! */
+    C_KZG_OK = 0x80000000,  /**< Success! */
     C_KZG_BADARGS, /**< The supplied data is invalid in some way. */
     C_KZG_ERROR,   /**< Internal error - this should never occur. */
     C_KZG_MALLOC,  /**< Could not allocate memory. */

Or this, which I think it pretty ugly but it's more clear what's going on:

/**
 * The common return type for all routines in which something can go wrong.
 */
typedef uint32_t C_KZG_RET;

/**
 * The possible return values for C_KZG_RET.
 */
enum {
    C_KZG_OK = (C_KZG_RET)0,
    C_KZG_BADARGS = (C_KZG_RET)1,
    C_KZG_ERROR = (C_KZG_RET)2,
    C_KZG_MALLOC = (C_KZG_RET)3,
};