Per function host config

I have grouped several functions (with service bus topic triggers) into one function app, but I need some of the functions to have different maxConcurrentCalls settings (among other settings). Is it possible to have separate host configs per function?

#### Repro steps

Provide the steps required to reproduce the problem

1. Create function 1, doesn't require maxConcurrentCalls to be set (so uses default value)

2. Create function 2, but requires a different maxConcurrentCalls value

#### Expected behavior

Be able to set the maxConcurrentCalls settings (among other settings) per function

#### Actual behavior

Settings are defined globally in host.json

#### Known workarounds

-

#### Related information

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Start by reviewing the host.json configuration described in the issue and the host/runtime entry points that apply its settings. Determine how service bus topic-triggered functions are grouped into an app and where per-function configuration could be represented. Done means functions in one app can use distinct maxConcurrentCalls values and other applicable settings without changing global defaults.

Written by the indexing model from the issue text.