[FR] Connector should push message back to rabbitmq if function call fails

[jebos]

Is your feature request related to a problem? Please describe.
In case openfaas is busy (timeout), or there is an error (response !=2xx), it seems required that the connector pushes the message back to the queue in order to avoid data to be lost.

Describe the solution you'd like
As mentioned, I think it would be nice if the message is pushed back to the queue, it may seem also a good idea to throttle execution in general (or in this case) to avoid highload loops, in case openfaas is not available.

Describe alternatives you've considered

Additional context
100.000 messages in a queue that shall be pushed through one single, not scaleable function (due to system limits and cost contrains - but not time constrains)

[Templum]

As offered earlier, will implement this feature. Follow this issue for any progress.

[Templum]

Created Draft PR #91, this feature will be made available to V1

[Templum]

@jebos would you willing to try out the change? Implementation and local testing were concluded.
And I plan to later merge the PR, which should make it available through the following tags develop & v1.

Please also see the following section, as this is the major difference between v0 & v1