Move template provider out of create_message!() macros.

[ndouglas]

Summary

Closes #72.

Definition of Done

• [x] Code is:
  • [x] written in idiomatic Rust
  • [x] commented where necessary
  • [x] documented where beneficial
  • [x] follows the project's coding standards, and
  • [x] is efficiently structured for maintenance and scalability.
• [ ] Changes are benchmarked to ensure they do not degrade performance.
• [x] Comprehensive unit tests cover new functionality and changes.
• [ ] Integration tests cover features with user stories where possible.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.53%. Comparing base (a62a5e3) to head (f8eb7ce).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #73 +/- ## ========================================== + Coverage 97.35% 97.53% +0.18% ========================================== Files 16 16 Lines 453 487 +34 ========================================== + Hits 441 475 +34 Misses 12 12 ```

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