Allow referring to a capture via `capture.<key>`

[rhrazdil]

It may be more user-friendly and less error prone to refer to captured state via capture identity, instead of having to use a pipe.

Consider the following nmpolicy snippet:

spec:
  capture:
    ethernets: interfaces.type=="ethernet"
    ethernets-up: capture.ethernets | interfaces.state=="up"

It would be nice if the pipe wasn't required to access a captured state, and allow referring to it directly by a path, like so:

spec:
  capture:
    ethernets: interfaces.type=="ethernet"
    ethernets-up: capture.ethernets.interfaces.state=="up"