This commit includes a major update to the .NET framework from `net7.0` to `net8.0` across all projects. It also introduces significant enhancements to the FedEx API integration, including new classes and interfaces for authentication and address validation. The `Microsoft.Extensions.Hosting` and `Microsoft.Extensions.Http.Polly` packages have been updated to version `8.0.0` and `8.0.4` respectively. The `FedExAddressValidationProvider` class now uses the FedEx Web Services client for address validation, with a note that this SOAP-based service will be disabled on August 31, 2024, and replaced with FedEx RESTful APIs. The `FedExApiOptions` class now includes a `Url` property for switching between development and production URLs for the FedEx API. The `FedExAuthClient` and `FedExAddressValidationApiClient` classes have been updated to handle token refreshes and API alerts respectively. The `AddressValidationResponse` class now includes a `IsValidAddress` method for validating addresses.

This commit includes a significant overhaul of the codebase, with numerous updates, additions, and deletions. Key changes include:

- Updated `.editorconfig` and `settings.props` files with new rules and warning suppressions.
- Enhanced several files with `cancellationToken` parameter and improved documentation.
- Added `ApiError.cs` for API error representation and updated `FedExApiOptions.cs` and `README.md`.
- Removed and replaced several files related to address validation with a new implementation.
- Refactored namespaces and updated methods for better organization and clarity.
- Added new interfaces, classes, and methods for FedEx Rates and Transit Times client.
- Added numerous classes for handling FedEx shipping rates requests and responses.
- Updated several classes for better null safety and improved retry policy.
- Removed unused `using` directives and updated project references.
- Refactored test classes to accommodate the new changes and improve test coverage.

These changes aim to improve the overall code quality, enhance the functionality of the library, and provide a more robust implementation for address validation and FedEx Rates and Transit Times client.

Updated the `next-version` in `GitVersion.yml` from `3.11.0` to `4.0.0` and the `VersionPrefix` in `settings.props` from `3.0.0-preview1` to `4.0.0-preview1`. These changes indicate the next software version and preview release version respectively.

Updated the description and package tags in the Nuget Package Settings for `EasyKeys.Shipping.FedEx.Abstractions.csproj`, `EasyKeys.Shipping.FedEx.AddressValidation.csproj`, and `EasyKeys.Shipping.FedEx.Rates.csproj`. The descriptions now include specific API details and the package tags have been updated to reflect the corresponding FedEx API versions and functionalities.

Updated the .NET version from 7.0.x to 8.0.x in the GitHub Actions workflows. This change affects `feature.yml`, `master-pr.yml`, and `master.yml` files. The `dotnet-version` parameter in the `actions/setup-dotnet@v1` action has been updated accordingly. This ensures that the workflows now run with .NET 8.0.x.