Example: New Kind generation

[tirthct]

This PR contains the output of the TRex code generator (see #27). In this example, a new Kind called "Subscription" is generated.

This PR will remain online as an example of a new Kind. It shows where various pieces need to be wired together, all of which are described in this project's README.

[markturansky]

This looks very good. Some suggestions:

1. extract the generator stuff and README to a separate PR (or extract the generated Subscription stuff from this one)
2. put generated "Subscription" stuff in another PR.
3. provide link in README where you explain usage of the generator to the sample PR with everything generated.

we merge the generator parts, but leave open the example PR with notes that explain what/why. Future improvements to the generator (to remove those remaining manual bits) can be generated anew for the open example PR.

[tirthct]

This looks very good. Some suggestions:

1. extract the generator stuff and README to a separate PR (or extract the generated Subscription stuff from this one)

2. put generated "Subscription" stuff in another PR.

3. provide link in README where you explain usage of the generator to the sample PR with everything generated.

we merge the generator parts, but leave open the example PR with notes that explain what/why. Future improvements to the generator (to remove those remaining manual bits) can be generated anew for the open example PR.

Separated out generator stuff in MR #27