feat(fcm): Set FCMOptions for MulticastMessage

[FZambia]

Hello!

Currently it's impossible to set FCMOptions for MulticastMessage. While for example in Python admin SDK this is possible. And in Java admin SDK. This pull request fixes this.

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[FZambia]

CLA signed

[lahirumaramba]

Hi @FZambia Thank you for your contribution. Please change the base branch to dev before we can start the review process.

https://github.com/firebase/firebase-admin-go/blob/master/CONTRIBUTING.md#want-to-submit-a-pull-request

Make sure to create all your pull requests against the dev branch. All development work takes place on this branch, while the master branch is dedicated for released stable code. This enables us to review and merge routine code changes, without impacting downstream applications that are building against our master branch.

[FZambia]

Hello team, could you please tell when this will be released?

[FZambia]

@lahirumaramba - any updates?

[FZambia]

Guys, at least explain what prevents you from merging this?

[chong-shao]

LGTM But I'd like to ask for waiting until @lahirumaramba 's review

[sebastianbuechler]

This would be nice to finally have in the package. @lahirumaramba can you please have a look at this minor change as it's only two lines of code and almost a year open?

[FZambia]

@lahirumaramba kindly reminding 🙏

[lahirumaramba]

Thanks folks! This change will be included in the next release

[FZambia]

Nice, many thanks!