Expose available StatementSupports from StmtContext

A number of inference interactions, notably implicit inputs and YANGTOOLS-1338 require acquiring the appropriate statement support for a specific statement from another statement's support class.

Introduce a new StatementSupportNamespace, which will be automatically populated by the reactor's SourceSpecificContext to reflect which statements are available. It needs to only support the get() operation based on a QName which is the statement name.