Level 2, 3, and 4 support

[DemiMarie]

This is a feature request for level 2, 3, and 4 support.

[dwrensha]

(For ease of cross-reference, here is a definition of the levels.)

The main details for level 2 are described in persistent.capnp. We should be able to copy whatever C++ does to get this stuff to work. I'm not sure when I personally might find time to work on this; others should feel welcome to step up.

Levels 3 and 4 are farther away. Currently, not even the C++ implementation supports these features.

[dwrensha]

@DemiMarie If you have any particular use cases in mind, I'd be curious to hear about them.

[insanitybit]

I'm considering this lib as a fairly core component of some services I'm building (haven't landed on anything 100% yet but it looks promising). I'm still learning how the cap system works so if any of this is wrong/ not helpful, feel free to say so or ignore.

Personally, I'm mostly interested in Level 2/3 support. I could see myself wanting Level 3 for cap delegation - specifically I have some pub sub stuff where I might have:

• Producer Service
• Subscription Service
• Consumer Service(s)

Where consumers register with the Subscription service, and the Subscription service provides the Producer service with the objects/ caps to talk to the Consumers. This is my current architecture but I haven't built the RPC component out.

I do not foresee needing level 4 support and I could get by happily with level 2 for a long time I think.

My plan originally, before learning of this cap system, was to rely entirely on mTLS.