src-cli: Implement new campaigns workflow

[mrnugget]

Docs/Schema in PR #10921

Tasks

• Implement src campaigns apply (see this comment https://github.com/sourcegraph/sourcegraph/pull/10921#issuecomment-648599857)
• Validate campaign spec YAML according to schema
  • I think duplicating that schema between here and src-cli is fine for now. If we can find a better solution that has src-cli fetch the schema from Sourcegraph: great.
• Create changesetSpecs with the createChangesetSpec mutation (see schema here), remembering their IDs
• Create campaignSpec, using the IDs as input
• Print preview URL if user doesn't want to apply
• Call applyCampaign if user wants to apply

Plus: clean up the code, possibly add tests, move code to smaller packages instead of the ./cmd folder, etc.

First deliverable

A draft PR that implements the new command and flow for us to test to validate how the UX feels.

[marekweb]

Dear all,

This is your release captain speaking. 🚂🚂🚂

Branch cut for the 3.19 release is scheduled for tomorrow.

Is this issue / PR going to make it in time? Please change the milestone accordingly.
When in doubt, reach out!

Thank you