Is your feature request related to a problem? Please describe.

For backwards compatibility with existing logs I need to send logs with textPayload instead of jsonPayload, and to do that I need text_payload_key, see https://github.com/fluent/fluent-bit/blob/master/plugins/out_stackdriver/stackdriver.c#L3235

Describe the solution you'd like

Add textPayloadKey property to stackdriver.

Additional context

No response