fix(firestore): deprecate databaseURL in favor of databaseId

[russellwheatley]

Description

• Updated user facing code to allow both databaseURL & databaseId. databaseId takes precedence over databaseURL if both are input. Otherwise, they have the same value (if one or the other are selected).
• Refactored method channel, platform and web implementations to use databaseId naming.
• Wrote unit test.
• We can update pigeon and native code to use databaseId naming in a future PR, perhaps when databaseURL is removed.

Related Issues

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Breaking Change

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[exaby73]

LGTM; just a couple of questions