Add time validator

Implement a new provider::validatefx::time validator.

Description:

• Validate that a string is a time in HH:MM[:SS] format.

Acceptance criteria:

1. Subtask: Implement time validator in internal/validators
2. Subtask: Add unit tests for time validator
3. Subtask: Add time function to internal/functions package
4. Subtask: Add unit tests for time function
5. Subtask: Register time in registry.go
6. Subtask: Add documentation in examples/functions/time/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf