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.

test/load/fixtures.sql

@@ -1,5 +1,7 @@
 CREATE ROLE postgrest_test_anonymous;
+CREATE ROLE postgrest_test_author;
 GRANT postgrest_test_anonymous TO :PGUSER;
+GRANT postgrest_test_author TO :PGUSER;
 CREATE SCHEMA test;
 
 -- PUT+PATCH target needs one record and column to modify
@@ -31,10 +33,18 @@ CREATE TABLE test.roles (
   character TEXT
 );
 
+CREATE TABLE test.authors_only ();
+
 CREATE FUNCTION test.call_me (name TEXT) RETURNS TEXT
 STABLE LANGUAGE SQL AS $$
   SELECT 'Hello ' || name || ', how are you?';
 $$;
 
-GRANT USAGE ON SCHEMA test TO postgrest_test_anonymous;
+GRANT USAGE ON SCHEMA test TO postgrest_test_anonymous, postgrest_test_author;
 GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA test TO postgrest_test_anonymous;
+
+REVOKE ALL PRIVILEGES ON TABLE
+      authors_only
+FROM postgrest_test_anonymous;
+
+GRANT ALL ON TABLE authors_only TO postgrest_test_author;