nix: add loadtest with unique JWTs

This loadtests the jwt decoding logic. For this it adds an optional
`-k`(kind) parameter to `postgrest-loadtest` and
`postgrest-loadtest-against`.

Old kind (default):

```
postgrest-loadtest -k mixed
postgrest-loadtest-against -k mixed
```

New kind:

```
postgrest-loadtest -k jwt
postgrest-loadtest-against -k jwt
```

Internally it uses a dynamically generated targets file using python
which looks like:

```
GET http://postgrest/authors_only
Authorization: Bearer <jwt>

GET http://postgrest/authors_only
Authorization: Bearer <another-jwt>
...
```

Then this is used to run vegeta with the `-lazy` option.

.github/workflows/test.yaml

@@ -126,11 +126,15 @@ jobs:
         id: get-latest-tag
         with:
           prefix: v
-      - name: Run loadtest
+      - name: Run loadtest (mixed)
         run: |
           postgrest-loadtest-against main ${{ steps.get-latest-tag.outputs.tag }}
           postgrest-loadtest-report >> "$GITHUB_STEP_SUMMARY"
 
+      - name: Run loadtest (jwt)
+        # TODO generate a report for this https://github.com/PostgREST/postgrest/issues/4022
+        run: |
+          postgrest-loadtest-against -k jwt main ${{ steps.get-latest-tag.outputs.tag }}
 
   flake:
     strategy: