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.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[kirmerzlikin]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[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()

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[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