feat(fcm): Added support for specifying analytics label in notifications

[RannyRanny]

Solves #88

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[RannyRanny]

I signed it!

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[hiranya911]

Thanks @RannyRanny for putting this together. We need a couple of new types in this PR and test coverage.

[RannyRanny]

@hiranya911 Added new classes and coveder tests

[hiranya911]

Thanks @RannyRanny. Looks pretty good. Just a few changes/improvements needed. Then we can merge.

[RannyRanny]

@hiranya911 Added requested changes

[hiranya911]

Sorry, but it looks like this needs a few more updates. The main points are:

1. Add the license header to all new source files.
2. Make sure there's a newline at the end of each file.
3. AnalyticsLabel is optional, and therefore should allow null.

[hiranya911]

@egilmorez can you look at the documentation bits of this PR?

[hiranya911]

LGTM :+1

[RannyRanny]

@hiranya911 This is my first PR for open source. Can U please say when this will be available to use in NuGet package?

[hiranya911]

@RannyRanny most likely next week.