expose global state namespace in configuration using field reference

this idea is not yet fully matured, this is an early "brain dump" on the idea for discussion. this idea resulted from a discussion with @jakommo about having the ability to keep information from a specific event that could be passed on to following events.

similar to the idea of the `[@metadata]` field reference feature, we could expose a global state or stream state/context namespace through field reference using, for example, `[@context]` which would access a global "per stream" object that could be used to get/set values using the field reference syntax in the config.

This would allow users to use a global state without having to write custom plugin scripts and keep this in the config using the same syntax and logic using conditionals etc.

This could also be used to expose some of the upcoming stream identity data using @guyboertje work on this? (can you please link the relevant issue(s) here?