api: list-zones: do not send auth header

[natalie-o-perret]

/zone is public but the server enforces IAM on authenticated requests, so a DBaaS-only key gets 403 for no reason.

In _call_operation, when the operation is list-zones, the request is sent via a plain requests.request without credentials. No IAM check fires and all zones come back.

Snippet used to verify:

from exoscale.api.v2 import Client

client = Client(key=key, secret=secret, zone="ch-gva-2")
result = client.list_zones()
zones = result["zones"]
print(f"ok: {len(zones)} zones")
for z in zones:
    print(f"  - {z['name']}")

Before (master, DBaaS-only key):

exoscale.api.exceptions.ExoscaleAPIAuthException: Authentication error 403: {"message":"Invalid request signature"}

After (this branch, same key):

ok: 8 zones
  - ch-gva-2
  - ch-dk-2
  - at-vie-1
  - de-fra-1
  - bg-sof-1
  - de-muc-1
  - at-vie-2
  - hr-zag-1

Related: exoscale/egoscale#767, exoscale/exoscale-sdk-java#14

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