[Test-Proxy] Customizable Default Matcher, Conditional Matching

The ask here is essentially:

> The standard test matcher has all the capabilities I need, but I need to _tweak_ some of them.

@JoshLove-msft wants to be able to customize the headers that are matched on.
@HarshaNalluru wants the ability to specify "conditional" matching. The ask originally had an example of a request that looks something like this:

```json
{
"requestUriRegex" : "https://login.microsoftonline.com/azuretenantid/oauth2/v2.0/token"
"match-conditions" : {
"match-body" : "false"
"header-list-to-not-match" : [ "Some-Dynamic-Header" ]
}
}
```

`requestUriRegex` could be a URI or a regex. It's intended to be used as a

> only apply these specific conditions if the request URI looks like this.

In terms of conditional matching, we should probably allow parts of the HTTP request to trigger the matching. I can see a specific `Header` or `Body` needing the same behavior.