Add ability to resolve snapshot latest date

[khlopko]

Closes issue: https://github.com/swift-actions/setup-swift/issues/673

Implements resolution of development snapshots either by explicit date or by fetched tags from swiftlang/swift repo (similar to how Swiftly does this) by setting "main-snapshot" or "X.Y-snapshot" to swift-version parameter. It relies on setting GH_TOKEN value in env from secrect.GITHUB_TOKEN to avoid hitting limits.

Unites package information building under getPackage function. Includes tests for new functionality, including snapshot resolution with cached GitHub responses. Updated README with new options for versions and adding token.