feat: add an initial set of execution steps

[rodesai]

This patch adds an initial set of execution step nodes. Subsequent patches
will have schemakstream/table build the execution tree as it goes along, and
then we'll move calls to streams into the implementations of ExecutionStep.build.

All execution steps implement ExecutionStep, which along with supporting a few
common properties (step id and schema), includes a method called build(), which
will eventually get called to build the streams app (as described above).

[agavra]

Don't all of the ExecutionStep classes need to be serializable somehow? Or will that be addressed in a future patch? Otherwise this LGTM - nice to have this PR introduce just boilerplate.

[rodesai]

Don't all of the ExecutionStep classes need to be serializable somehow? Or will that be addressed in a future patch? Otherwise this LGTM - nice to have this PR introduce just boilerplate.

@agavra yes this (along with patches to serialize expressions) is coming. I wanted to define the steps with this PR, then start submitting PRs to:
- build the plan from (Grouped)SchemaKStream|Table
- move building of the streams app into the step classes
- serialize the steps (and expressions) to the command/config topic for interactive/standalone mode.

[agavra]

LGTM! Thanks for the clarifications on next steps @rodesai