Add port configuration for MockService and MockServer

[surpher]

📝 Summary of Changes

The changes in this PR introduce a new MockService initializer that accepts the port number on which to run the MockServer. This will allow tests to always run on the same port between test runs.

Along with the PactSwift enhancements described above, the following has been done:

• Improves MockService tests
• Update SwiftLint Phase Script for v0.41.0
• Update GitHub Actions workflows

⚠️ Items of Note

• ISSUE: aarch64 - Xcode 12.2 requires binaries to contain aarch64 slices. This means that the script that builds pact_mock_server.a needs to build for aarch64 and x86_64 and create a fat binary containing both slices. Though the cargo build fails when even when using Rust nightly toolchain. It fails when building the pact_mock_server.a binary for aarch64 architecture from Rust code (building on x86_64 machines) due to pact_mock_server's dependencies' fail to build. Please focus on iOS platform when using Carthage or preferrably use Swift Package Manager to bring PactSwift into your project. arm64 architecture for macOS has been removed with Exclude arm64 for macos #53 until better support is available.
• Port remains random if not provided when initialising MockService
• Only port numbers above 1200 are valid and if port is not available it should fail with MockServerError(code:)

🧐🗒 Reviewer Notes

• Resolves Add port configuration for MockServer #50

🔨 How To Test

• CI passes
• Setup a test signing the URL with HMAC (optional)

[georgescumihai]

I left some remarks, this change should be enough to fix #50