The custom sub-set interfaces in init/join phases as a pattern
have isolation benefits - e.g. when a consumer imports these
but we don't want them to be able to call all methods from
the original object that satisfies a complete interface.

On the other hand these sub-set interfaces under phases/init
and phases/join are private.

Expose a couple of new common interfaces:
- InitData from phases/init/data.go
- JoinData from phases/join/data.go

Use these interfaces in init/join phases accordingly instead
of the sub-set interfaces.

Use compile-time type assertion to verify that these
interfaces can be satisfied by init.go's initData and
join.go's joinData.

Add NO-OP objects called testInitData and joinInitData
that can be used for unit testing if embedded.