Add event tracking for project parse/load time

[kwigley]

resolves #2823

Description

Added new event named load_project. The event includes high level tracking of project parse times. This will help track and inform efforts to increase dbt performance 👍

I decided to move the timing info into a dataclass to help me out a little when creating the event context. I can see us surfacing this info elsewhere as well.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[jtcohen6]

I've been testing this out, and it's looking great to me! Excited to dig into the data.

I don't think the event fires for the actual dbt parse command, which feels right—it's not really an invocation, and the explicit purpose of that command is to collect more detailed performance info than what's available here.

I forgot our convention of sticking the invocation_id in the structured event label. That's ok, I'm happy we're sticking it there and right in the context as well, and I don't mind the duplication.

[kwigley]

@jtcohen6 awesome! I consciously didn't add the event to the parse task, so that's good to hear :) I'll note deets like this in the PR desc going forward.

Ya, I didn't realize the label convention until after we merged the event schema. We can always clean up if we ever iterate on it 👍