Default behavior todos

[jack-berg]

Followup to #426.

Common themes:

• For all properties in a SDK extension plugin, set defaultBehavior: ignore.
• For properties with complex types, where those types have defaults of their own for each property, set defaultBeahvior: default values as described in <type> are used

There weren't too many default behaviors that were completely missing. Mostly just dots that needed to be connected.

There was only one case where the defaults were unspecified and not obvious - the jaeger remote sampler. I made endpoint and initial_sampler required, and set the defaultBehavior: 60000 is used for internal (the default in java but not defined in the spec). This case isn't too important because we marked jaeger remote sampler as experimental in #363.

[jack-berg]

Closing in favor of #439.