Snippets for new Auth features

[samtstern]

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Hey there! So you want to contribute to a Firebase SDK? Before you file this pull request, please read these guidelines:

Discussion

Based on: https://github.com/firebase/snippets-node/pull/94

Testing

Depends on #339 and #344

API Changes

N/A

[samtstern]

@rsgowman thanks for the comments! I feel dumb for making those syntax errors, but they also show the risk of writing snippets when you expect the CI to fail, I didn't even read the failure logs!