Provide offset to placeholder in apphost pack

[nguerrera]

Today, the SDK does a search and replace in the apphost, to find the placeholder. However, the offset to the placeholder is fixed for any given apphost pack. It would be cleaner if this offset could be determined when the apphost template is built and described in a simple sidecar file.

The code that does the search fails on some filesystems today, due to MAP_SHARED not being implemented. We could implement the search a different way, but risk regressing perf for an operation that needn't be done on every end-user's build, and can be done once in our build.

cc @peterhuene @swaroop-sridhar @jeffschwMSFT

[nguerrera]

The idea of providing the offset was raised in the initial PR that implemented search and replace: dotnet/sdk#978 (comment)

We were missing the additional data point that the memory mapped code would fail on some filesystems, though.