Service tier for the request.
Forwarded as the service_tier field on the Fireworks chat completions
request when set. Pass 'priority' to opt into Fireworks' priority tier;
leave as None to use the default tier.
To use Fireworks' fast mode instead, select a fast-routed model; fast mode
is not controlled by this field. See Fireworks'
serverless product docs
for the current list of fast routers and tiers.
