Add Flow Action: "String Message Digest" for generating string-based hash digests

[anveshmupparaju]

This pull request introduces a new Flow Action named "String Message Digest", which enables users to generate cryptographic digests from input strings using the following algorithms:

• MD5
• SHA1
• SHA256

The action supports two output formats:

• Base64
• Hexadecimal

Key Features:

• Clean and extensible logic using a mapping structure for algorithm-output combinations.
• Designed for easy integration into Flow Designer for use in automation and integration scenarios.

Use Cases:

• Generating unique identifiers or checksums for strings.
• Verifying data integrity in integrations.
• Lightweight cryptographic operations.

Tested Scenarios: All supported algorithm-output combinations.

[github-actions]

✅ Valid PR for ActionPack

Thank you for your contribution. This PR complies with the CONTRIBUTING.md.
A maintainer will review this shortly. In the meantime, Happy Hacking!