Add OSLog support to SFSDKLogger

[TheAlmightyBob]

Summary

OSLog provides more modern console logging than the existing mechanism used by SFSDKLogger, with distinct log entries, status coloring, and filterable metadata.

The underlying CocoaLumberjack already supports it (in fact, it actually logs a warning if you're still using the old logger), so this PR simply provides the option to enable it. It still defaults to the existing behavior to be safe.

Details

• New useOSLog class property on SFSDKLogger. Defaults to NO. Must be set prior to creating logger instances.
  • (assumption is that you wouldn't want the logging mechanism to change mid-log anyway)
• If enabled, console logs will use OSLog to provide distinct log lines with metadata instead of the previous plain text logging method
• Updated the previously-empty SalesforceFileLoggerTestApp to log a few messages with/without OSLog

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @TheAlmightyBob to sign the Salesforce Inc. Contributor License Agreement.

[TheAlmightyBob]

I tried to sign the CLA and it returned an error. If I try again it says I already signed...

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.08%. Comparing base (d88e75b) to head (1d95755).
⚠️ Report is 9 commits behind head on dev.

Files with missing lines	Patch %	Lines
.../SalesforceFileLogger/Classes/Logger/SFSDKLogger.m	84.21%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #271      +/-   ##
==========================================
+ Coverage   61.39%   63.08%   +1.68%     
==========================================
  Files          21       21              
  Lines        1816     1834      +18     
==========================================
+ Hits         1115     1157      +42     
+ Misses        701      677      -24     

Flag	Coverage Δ
SalesforceFileLogger	70.55% <86.95%> (+10.16%)	⬆️
SalesforceHybridSDK	61.47% <ø> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../SalesforceFileLoggerTestApp/Classes/AppDelegate.m	63.63% <100.00%> (+20.77%)	⬆️
.../SalesforceFileLogger/Classes/Logger/SFSDKLogger.m	65.41% <84.21%> (+12.23%)	⬆️

... and 1 file with indirect coverage changes

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