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]

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