feat: add ai tasks migrations (#18359)

Adds database migrations required for the Tasks feature.

There's a slight difference between the migrations in this PR and the
RFC: this PR adds `NOT NULL` constraints to the `has_ai_task` columns.
It was an oversight on my part when I wrote the RFC - I assumed the
`DEFAULT FALSE` value would make the columns implicitly NOT NULL, but
that's not the case with Postgres. We have no use for the NULL value.

The `DEFAULT FALSE` statement ensures that the migration will pass even
when there are existing rows in the template version and workspace
builds tables, so there's no danger in adding the `NOT NULL`
constraints.

coderd/database/sqlc.yaml

@@ -147,6 +147,8 @@ sql:
           crypto_key_feature_workspace_apps_api_key: CryptoKeyFeatureWorkspaceAppsAPIKey
           crypto_key_feature_oidc_convert: CryptoKeyFeatureOIDCConvert
           stale_interval_ms: StaleIntervalMS
+          has_ai_task: HasAITask
+          ai_tasks_sidebar_app_id: AITasksSidebarAppID
 rules:
   - name: do-not-use-public-schema-in-queries
     message: "do not use public schema in queries"