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.
steve-chavez