[Evals] Built-in or integrated evaluation metrics

[ssbushi]

Is your feature request related to a problem? Please describe.
Evaluation metrics existing as a separate plugin that needs to be explicitly installed may be a hurdle for user.

Describe the solution you'd like
Some considerations:

• Provide genkitEvals plugin as part of the core SDK. (no need to do a separate npm i)
• Stretch, make some metrics integrated into the CLI to leverage a common platform. No need to port every metric to every runtime (instant parity). Considerations needed here are how to provide genkit primitives in CLI, or perhaps a workaround that references a CLI-metric from the runtime.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[ssbushi]

@chrisraygill FYI