feat: Add support for datetime expressions

[surpher]

📝 Summary of Changes

Changes proposed in this pull request:

• Adds a new DateTimeExpression example generator that handles date time expressions
  (Feature: Rust implementation of DateTime Expressions pact-foundation/pact-reference#180)
• Refactors DateTime example generator
• Fixes the env var's value for logging
• Refactors Date helpers
• Fixes copyright headers

💁 Example

.willRespondWith(
    status: 201,
    body: [
        "start": ExampleGenerator.DateTimeExpression(expression: "@ next hour", format: dateFormat)
    ]
)

📝 Notes

• PactSwift uses pact-rust as the core. When creating the Pact structure that's being passed to the rust core, the actual date value is irrelevant because rust core re-generates it based on the given expression. I've reached out to pact-rust team about thoughts on passing an optional reference date.

🔨 How To Test

Unit tests have been written.
Create a project where an ExampleGenerator.DateTimeExpression(format: expression: use:) is defined in a request's or response's body.
Run the test and verify the written Pact file contains the datetime generator with an expression.

[marcelinoagile]

Hi @surpher I created PR #85 with a change which is a nice to have IMHO