fix: auth_time emulator behavior should match production (auth_time is now set to user's last sign in time). (#3608)

[lovelle-cardoso]

Description

Fixes #3608 so that emulator sets auth_time the same way production does (auth_time should match lastLoginAt in seconds)

Scenarios Tested

Added unit test to src/test/emulators/auth/misc.spec.ts called "should populate auth_time to match lastLoginAt (in seconds since epoch)"

This unit test...

1. Registers a user
2. Exchanges a refresh token for a new token
3. Checks that the new token's auth_time matches the user's lastLoginAt in seconds.

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[lovelle-cardoso]

@googlebot I signed it!

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[lovelle-cardoso]

My commit author wasn't correct. So I fixed it here: #3611