Tracks implementing the MSFZ symbols format.

Required work:

• Update SymbolReader to add a new HTTP accept header when making a request to a symbol server: "Accept: application/msfz0"
• If the contents of the downloaded file starts with "Microsoft MSFZ Container" (UTF8 encoded) then it should be put in a separate subdirectory of the normal cache path. The separate subdirectory should be called "msfz0".
• Before SymbolReader downloads from a remote symbol server, it looks in local symbol caches. When searching a local symbol cache for a matching file, check both the current directory that gets searched as well as the new "msfz0" subdirectory if it exists. If either directory exists and contains a PDB file, the file can be used.