2.10 Features: Stream manager improvements

[JR-Morgan]

A prerequisite for #85
A rewrite of the Stream Manager to separate the commit selection and receiving functionality from the UnityEditor namespace.

• Refactored Account/Stream/Branch/Commit selection - now they are in nice reusable OptionSelection<T> wrappers that can be serialized nicely.
• Added new Speckle Receiver component as direct replacement for Stream Manager.
  • With some improvements over the old Stream Manager
  • Searchable stream/branch/commit menus
  • Serialised selection
  • Stream preview
• Added new Speckle Sender component to allow for editor sending (with selection).
• In future, these new components will also replace the runtime receiving components. see Consolidate Editor and Runtime components #85

TODO:

• Update 2.10 core/objects
• Bump package version
• Check with @haitheredavid about conflicts?