Replace MD5 hasher with SHA-256

[Whning0513]

Summary

• replace hashlib.md5 with hashlib.sha256 in framework.utils.hasher
• update the helper docstring to describe the new stable SHA-256 hash
• add a regression test that checks the digest content and 64-character output

Testing

• python tf_quant_finance/experimental/pricing_platform/framework/utils_test.py

Closes #109

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