feat: add SHA-2 family of hash functions

[AliAlimohammadi]

Description

Implements the full SHA-2 family of cryptographic hash functions (FIPS PUB 180-4) as pure-Rust, dependency-free one-shot functions:

Function	Digest	Word	Rounds
sha224	224 b	32	64
sha256	256 b	32	64
sha384	384 b	64	80
sha512	512 b	64	80
sha512_224	224 b	64	80
sha512_256	256 b	64	80

SHA-2 is a family of Merkle–Damgård hash functions with a Davies–Meyer compression step. A 32-bit engine (SHA-256/224) and a 64-bit engine (SHA-512/384/512-224/512-256) share the same structure but differ in word size, number of rounds, initial values, and block size.
Reference: NIST FIPS PUB 180-4

This PR also removes the existing sha256.rs, which provided only a stateful SHA-256 struct. All six SHA-2 variants are now covered in a single sha2.rs module. The hashing_traits.rs test that depended on sha256.rs has been removed along with it, as SHA256 and get_hash_string are no longer part of the public API.

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Breaking: sha256.rs is deleted. The SHA256 struct and get_hash_string function it exported are removed. The replacement one-shot sha256(msg: &[u8]) -> [u8; 32] covers the same algorithm.

Checklist

• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked CONTRIBUTING.md and my code follows its guidelines.

Note on clippy::big_endian_bytes: sha2.rs carries #![allow(clippy::big_endian_bytes)] at the top of the file. This lint is from clippy's restriction group and flags all big-endian byte conversions. SHA-2 is defined by FIPS 180-4 to use big-endian byte order throughout; switching to little-endian would produce wrong digests. The allow is intentional and documented in the file.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.97%. Comparing base (1c7738e) to head (88159f7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
- Coverage   96.06%   95.97%   -0.10%     
==========================================
  Files         388      388              
  Lines       29468    29427      -41     
==========================================
- Hits        28309    28243      -66     
- Misses       1159     1184      +25     

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[AliAlimohammadi]

@siriak, this is ready to be merged.