Add educational SHA-256 implementation

[AElnamaki]

Description
Educational SHA-256 implementation in Python with type hints and doctests.

References
-https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf

Checklist
I have read CONTRIBUTING.md.
This PR is all my own work.
All pre-commit checks pass.
The algorithm includes type hints.
The algorithm includes doctests that pass.
Filename follows repository naming conventions.
Only one algorithm file added.

[algorithms-keeper]

Closing this pull request as invalid

@AElnamaki, this pull request is being closed as none of the checkboxes have been marked. It is important that you go through the checklist and mark the ones relevant to this pull request. Please read the Contributing guidelines.

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