Replace OctopusRepositoryExtensions with methods that reuse RootResource

[rosslovas]

IOctopusAsyncRepository and IOctopusRepository have extension methods ForSpace and ForSystem which return a new repository with a different scope. This had the unintended consequence of discarding a cached RootResource if the repository had one, meaning the repository with the changed scope will have to do a fresh HTTP request to /api to get the root resource that could potentially have been avoided.

This PR replaces these extension methods with methods on the repository themselves, which call a new constructor that is able to keep track of the cached RootResource if there was one, solving this problem.

While I would expect most consumers to be unaffected by this change, it is technically a breaking change for certain niche scenarios. A few examples:

• a consumer had declared their own ForSpace/ForSystem extension methods, but now method resolution prefers the actual instance methods over those extension methods; solvable via renaming the extension method(s) to avoid the conflict
• a consumer created a class that inherits from a repository that was declaring its own ForSpace/ForSystem method(s); these now must specify the override keyword
• a consumer created a custom class that implements IOctopus(Async)Repository; now they have two new methods to implement themselves

As such, I will bump the major version here with +semver: breaking.