add sendMessage to the FCMProvider protocol so it can be mocked in tests

[FredericRuaudel]

Hello !

This PR declare the sendMessage in the FCMProvider protocol so we can mock it by using the config.prefer mechanism of Vapor 3 like this:

services.register(FCMMock.self)
config.prefer(FCMMock.self, for: FCMProvider.self)

[m-maillot]

I vote for this PR :)

[FredericRuaudel]

Thanks for the quick merge! 🙏

[MihaelIsaev]

@FredericRuaudel you're welcome! Thanks for the useful pull request! 🚀