Add XTEA block cipher implementation

[PentesterTN]

Describe your change:

Added an implementation of the XTEA (eXtended Tiny Encryption Algorithm) block cipher. XTEA is a lightweight Feistel cipher published by Wheeler and Needham in 1997 as a fix for weaknesses in TEA. It operates on 64-bit blocks with a 128-bit key and 64 rounds. Still shows up in embedded systems and game networking code because of its tiny footprint.

The implementation includes both encrypt and decrypt functions, uses only struct from stdlib, and has full doctest coverage with input validation.

Reference: https://en.wikipedia.org/wiki/XTEA

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".