Add default storage location

[vishweshbankwar]

• Adds StorageFolder as option to provide location for saving telemetry offline.
• If StorageFolder is not provided then following default base locations will be used:

For Windows:

• %LOCALAPPDATA%\Microsoft\ApplicationInsights
• %TEMP%\Microsoft\ApplicationInsights

For NonWindows:

• %TMPDIR%/Microsoft/ApplicationInsights
• /var/tmp/Microsoft/ApplicationInsights
• /tmp/Microsoft/ApplicationInsights

Open Question:

1. Should we use same folder location for traces/logs/metrics or separate folder for each (e.g. %LOCALAPPDATA%\Microsoft\ApplicationInsights\traces, %LOCALAPPDATA%\Microsoft\ApplicationInsights\metrics).

• Having separate location for each type will result in offline telemetry transmitted by their respective exporters. TraceExporter will not be able to transmit telemetry stored offline by MetricExporter.

NOTE:: Using this default base location, a subdirectory will be created by persistent storage based on application.
E.g.
Telemetry for AppA will be stored in %LOCALAPPDATA%\Microsoft\ApplicationInsights\aedb383c9dd16b651394c322cf3c5eafccd12daf81246dd930e905661a8c4edd

and AppB will be stored in
%LOCALAPPDATA%\Microsoft\ApplicationInsights\abdcc83c9dd16b6cbdbbd1246dd930e905661a8c4edd

TODO;
Add tests
Add security for directory in windows

[reyang]

LGTM.