Currently the `postgrest-loadtest -k jwt-*` loadtests use the OPTIONS method,
which is good for isolating the perf changes for JWT auth.
But using GET is more accurate for actual perf improvements,
since it actually hits the database.
This adds a method argument for postgrest-loadtest, so GET can be used like:
```
postgrest-loadtest -k jwt-hs --method GET
```
CI is not changed, this is mostly useful for manually checking variance.
steve-chavez
99b8246d6e