fix: Change string.Format style to string interpolation

[msohailhussain]

Summary

• Changed string.Format to string interpolation format. string.Format is bit expensive operatino.
• Added thread safe support for ForcedVariation.

Test plan

• FPS shouldn't break it.

[msohailhussain]

ThreadSafe dictionary.

TODO: Add unit test

[mikeproeng37]

Nice, are you able to profile and show some numbers along with where/how you ran it? Just curious

[msohailhussain]

Sure, I will share results.

[msohailhussain]

@mikeng13 I tested it separately and don't see any major difference. For more than 150K+ requests it took 0.5 seconds for this log.
string.Format(@"{0} Variation ""{1}"" is mapped to experiment ""{2}"" and user ""{3}"" in the forced variation map", Logger.DEBUG, experimentKey, variationKey, i.ToString());
and I see more or less the same time (avg 0.445 sec)
$@"{Logger.DEBUG} Variation ""{ experimentKey}"" is mapped to experiment ""{variationKey}"" and user ""{i.ToString()}"" in the forced variation map";

[mikeproeng37]

Thanks