Reintegrate Workspace Prebuilds feature branch into `main`

[dannykopping]

Development on the Workspace Prebuilds feature has been taking place in a feature branch (dk/prebuilds). We opted for this approach for rapid development in service of getting fast customer feedback; we have an Early Access release being shared with select customers this week.

While we await feedback, we have begun the process of decomposing and merging the feature branch code back into main, and hardening the code while we do so.

coder/coder

• Add support for system users to the database #515 -> chore: add prebuilds system user coder#16916
• Add support for prebuilds to the database #514 -> feat: add migrations and queries to support prebuilds coder#16891
• Recognise and persist prebuild definitions on template import #507 -> feat: persist prebuild definitions on template import coder#16951
• Provisioning New Prebuild Workspaces #513 -> feat: mark prebuilds as such and set their preset ids coder#16965
• Workspace Agent Prebuild Awareness and Authentication #511
• Claiming a Prebuilt Workspace for a User #512
• Prebuild Reconciliation Loop #510 -> feat: implement reconciliation loop coder#17261
• Prebuilds Metrics #509
• Enable Prebuilds #508

coder/terraform-provider-coder

• Add support for defining prebuilds on workspace_presets terraform-provider-coder#362 -> feat: allow presets to define prebuilds terraform-provider-coder#373
• Ensure the workspace agent can reuse its existing token when required terraform-provider-coder#364 -> feat: reuse agent tokens when a prebuilt agent reinitializes terraform-provider-coder#374

coder/observability

• Create prebuilds dashboard & alerts #561

coder/coder#16969 is our public RFC for this feature.