feat(fcm): Add missing WebpushFcmOptions entity (as per documentation)

[kirmerzlikin]

WebpushFcmOptions is described in documentation but is not included in Java client.

Corresponding issue - #294

RELEASE NOTE: Added a new WebpushFcmOptions API for configuring the features provided by the FCM SDK for Web.

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[kirmerzlikin]

@googlebot I signed it!

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[hiranya911]

Thanks for working on this @kirmerzlikin. Can you make sure the new class has the following public API:

class WebpushFcmOptions
     static Builder builder()
     static WebpushFcmOptions withLink(String link)
     class Builder
         Builder setLink(String link)
         WebpushFcmOptions build()

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[kirmerzlikin]

@hiranya911 done

[hiranya911]

Approved with a couple of suggestions. Please make those changes, and then I can merge. Also please add an entry to CHANGELOG file.

[hiranya911]

@egilmorez can you review the javadoc-related changes in this PR?

[egilmorez]

Looks great, thanks!

Just one nit on one sentence.

[hiranya911]

Thanks @kirmerzlikin. I can merge once you make the change suggested by Eric.

[kirmerzlikin]

@hiranya911 done