Add ConfigAwarePlaceholders class

[donaldgray]

This allows placeholder values to be sourced from IConfiguration in
addition to set values.

New Feature

Proposed Changes

I found myself writing a few DelegatingHandlers for appending headers for downstream calls using values from IConfiguration. This PR contains a new implementation of IPlaceholders.

This decorates and replaces the IPlaceholders instaces in the IoC container to allow the existing placeholders to be used (it checks these first). The idea is to have config like:

"UpstreamHeaderTransform": {
    "x-api-key": "{DownstreamService:ApiKey}"
}

I've added AddConfigPlaceholders() to IOcelotBuilder but I wasn't sure if that was the correct location, or if somewhere else would work.