Flex Consumption Plan: Allow per-function limits for concurrency

#### What problem would the feature you're requesting solve? Please describe.

Currently the Flex Consumption plan does not allow for fewer than 40 instances to be configured as a lower limit.

For functions like Service Bus Triggers, 40 simultaneous instances may result in overloading downstream databases or services.

At the individual function level, there's no way to set limits for concurrency.

#### Describe the solution you'd like

Ideally, we'd like the ability to configure maximum concurrency (across all instances) for individual functions.

At minimum we need the same ability to set lower caps for maximum instances like we're able to do with other Consumption plans today.

#### Describe alternatives you've considered

Currently the only options we see are continuing on a regular Consumption plan or migrating away from Azure Functions.

#### Additional context

Azure/azure-functions-host#912 is similar, but somewhat narrower in scope