* Add declarative Flow CLI support
Currently, declarative flows can be loaded by the runtime, but the CLI
still treats them as an experimental definition file instead of a
first-class Flow project shape.
With this PR, `crewai create flow --declarative` scaffolds a YAML-backed
Flow project, and `crewai run`, `crewai flow kickoff`, and `crewai flow
plot` can run against the configured definition.
This also lets crew actions reference reusable crew definition files or
folders and override their inputs from the Flow definition, so
declarative flows can compose existing declarative crews without
inlining everything.
* Address code review comments
