add didReceiveRegistrationToken to firebase_messaging on ios

[urbandove]

Currently on ios on app startup FCM throws an error that didReceiveRegistrationToken is not set up and when attempting to get a token using the getToken function it will often hang without returning a token. (When using await will hang and when using a callback will never call the callback)

Adding this has fixed the issue

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[urbandove]

Signed It

[googlebot]

CLAs look good, thanks!

[kroikie]

Hi @urbandove thank you for the PR. You are correct that we should implement didReceiveRegistrationToken. Could you also remove didRefreshRegistrationToken? That method has been removed and should not be used.

[urbandove]

@kroikie Done

Probably should bump the version of firebase_messaging with this change

[kroikie]

@urbandove yes please bump the version number as well as make a CHANGELOG entry, thanks!

[urbandove]

@kroikie Done

[kroikie]

@urbandove thanks! Looks good to merge.