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.
PR #2358 added a bulk insert to the loadtest. However this broke the regular insert test,
which just returned 400 Bad Request because of a missing PK value since. Adding the new
id column in the payload to the ?columns= argument fixes that.
steve-chavez