refactor(SPV-1231): admin endpoints group - Paymails API.

[mgosek-4chain]

Major Changes:

Admin API Enhancements:

• Implemented support for the paymails Admin API endpoints, complete with detailed documentation.

Documentation and Examples Updates:

• Updated code examples, the examples README, and the Taskfile to reflect the latest changes and improvements.

How to test 🤔

1. Go to examples dir cd examples
2. Set XPriv in examples_keys.go file.
3. Run the following commands in any order 😄:

• task create-paymail-as-admin
• task delete-paymail-as-admin
• task fetch-paymail-as-admin
• task fetch-paymails-as-admin

Pull Request Checklist

• 📖 I created my PR using provided : CODE_STANDARDS
• 📖 I have read the short Code of Conduct: CODE_OF_CONDUCT
• 🏠 I tested my changes locally.
• ✅ I have provided tests for my changes.
• 📝 I have used conventional commits.
• 📗 I have updated any related documentation.
• 💾 PR was issued based on the Github or Jira issue.

[github-actions]

Manual Tests

ℹ️ Remember to ask team members to perform manual tests and to assign tested label after testing.