Add the CommandDescriptor, and a CommandContext with only namespaceString, to api.h. Add the shared descriptor handle, and the SDK command descriptor base class and its adapter. run doesn't take an execution context yet; SERVER-137297 adds it. SERVER-137298 adds the remaining CommandContext fields.
Tests (unit, the handle driving the SDK adapter, following the existing stage handle tests):

• get_name, get_properties, and run round-trip through the handle and the adapter, and the handle takes ownership of returned buffers.
• Exceptions in run or getProperties come back as statuses.
• Existing stage extensions load unchanged.