Migrate to Contract classes for TargetDiscovery

[Jtango18]

⚠️ Does this change require a corresponding Server Change?
⚠️ If so - please add a "Requires Server Change" label to this PR!

Tipped off by the comment

    /// <summary>
    /// This type and the types it uses are duplicated here from Octopus.Core, because:
    /// a) There is currently no existing project to place code shared between server and Calamari, and
    /// b) We expect a bunch of stuff in the Sashimi/Calamari space to be refactored back into the OctopusDeploy solution soon.

This copies the Target Discovery files from Server and puts them in Calamari Contracts and replaces existing executions.

Ideally Server would be updated to leverage these instead of it's in-built classes too

[APErebus]

Looks good! I'm assuming the Context is serialized into a variable as JSON and sent across