**[IMPROVEMENT]** Port hex_to_int from C to Rust

[ishwarthecodddr]

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Description
This PR ports the hex_to_int utility function from C (src/lib_ccx/utility.c) to Rust (src/rust/lib_ccxr/src/util/hex.rs) as part of the ongoing Rust migration effort.
Changes
Created src/rust/lib_ccxr/src/util/hex.rs with the Rust implementation. Exposed the function via C-FFI in
src/rust/src/libccxr_exports/util.rs
Updated src/lib_ccx/utility.c to use the Rust implementation when DISABLE_RUST is not defined.

Changes :- Created src/rust/lib_ccxr/src/util/hex.rs with the Rust implementation.Exposed the function via C-FFI in src/rust/src/libccxr_exports/util.rs.
Updated :- src/lib_ccx/utility.c to use the Rust implementation when DISABLE_RUST is not defined.

Verification :- Ran cargo test -p lib_ccxr and verified that all tests passed, including the new
test_hex_to_int.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit 42d7509...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	7/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	86/86
Teletext	21/21
WTV	13/13
XDS	34/34

Congratulations: Merging this PR would fix the following tests:

• ccextractor --hardsubx 1a0302f7fd..., Last passed: Never

All tests passing on the master branch were passed completely.

Check the result page for more info.