Draft: initial scaffolding for SDK encryption

[Lance-Drane]

Needs to be completed before done:

[X] - basic scaffolding for encryption (Lance)
[] - encryption implementation (Marshall/Jesse/Andrew/etc.)
[] - e2e / integration test examples (Lance/Marshall/Jesse/Andrew/etc.)

(I don't really care too much about unit tests, but you can unit test the encryption function itself if you want)

To add the capability endpoints for a Service (which, IIRC, is just the get_public_key function), add it to IntersectSdkCoreCapability (https://github.com/INTERSECT-SDK/python-sdk/blob/candidate-0.9.0/src/intersect_sdk/capability/universal_capability/universal_capability.py), it should be annotated with @intersect_message . If we need additional state in the capability, we can modify the constructor for this capability and potentially the service as well.

Then you can call the capability with intersect_sdk.<YOUR_FUNCTION_NAME_HERE> as the operation from clients.

The actual encrypt/decrypt functions I would prefer to be separated out into utility functions in another file somewhere under the _internal directory, because both the Service and the Client will use them and users also have no reason to ever use this function themselves. I have marked the areas which need the inserted functions with #TODO comments, there should be one of each in client.py and two of each in service.py.