Injector and Extractor being different interfaces

[carlosalberto]

Currently a Propagator is expected to contain inject and extract operations, but OTEP 66 originally exposed different interfaces for them (namely Injector and Extractor), and some languages might be better offering as different interfaces as well.

Probably we can split them in the Specification, while mentioning that implementors can keep them in a unified interface (like most languages do at this moment) if that suits them.

[mwear]

@carlosalberto I can take this, as I think Ruby was one of the hold outs. We've introduced the propagator and composite propagator interfaces, but we build them from individual injectors and extractors. With some minor spec changes we can account for this variation.

[carlosalberto]

@mwear Thanks a lot, appreciated!

[carlosalberto]

Closing this via #578