v2/app: allow dbs to be defined outside svcs

[eandre]

Since we now accurately track database usages, we can
relax where they are defined. They can still only be used
from within services, to ensure permissions are mapped correctly.

[encore-cla]

All committers have signed the CLA.

[github-actions]

Test Results

    1 files  ±       0  152 suites   - 3   7m 2s ⏱️ - 10m 36s
540 tests +       8  537 ✔️ +     16  1 💤 +1  2 ❌  - 1 
540 runs   - 2 052  537 ✔️  - 2 044  1 💤 +1  2 ❌  - 1 

For more details on these failures, see this check.

Results for commit e754ede. ± Comparison against base commit 0be2cc4.

This pull request removes 15 and adds 23 tests. Note that renamed tests count towards both.

encr.dev/e2e-tests ‑ TestRun/encore_currentrequest
encr.dev/e2e-tests ‑ TestRun/et_override_user
encr.dev/e2e-tests ‑ TestRun/et_override_user_authdata
encr.dev/e2e-tests ‑ TestRun/experiment_local_secrets_override
encr.dev/e2e-tests ‑ TestRun/fallback_routes
encr.dev/e2e-tests ‑ TestRun/pubsub_method_handler
encr.dev/e2e-tests ‑ TestRun/pubsub_ref
test/svc ‑ TestRun
test/svc ‑ TestRun/encore_currentrequest
test/svc ‑ TestRun/et_override_user
…

encr.dev/e2e-tests ‑ TestEndToEndWithApp
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/POST_and_GET
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/Send_a_simple_request
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/app_metadata
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/cache
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/dependency_injection
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/empty_request
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/invalid_parameter
encr.dev/e2e-tests ‑ TestEndToEndWithApp/basic_requests/no_request,_response
…