MVP of persistence for the data connect emulator

[joehan]

Description

Starting to add support for data persistence between runs of the data conect emulator, as described in go/fdc-emu-import-api. This adds the emulators.dataconnect.dataDirectory field to firebase.json - when set, data will be saved to that directory and persisted between emulator runs.

Scenarios Tested

Ran a few CREATES, shut down the emulator, and verified they were still there on restart.

Follow up work

• Add a Clear Data admin endpoint that empties out this directory, and give VSCode users an easy button to trigger it.
• Proper Import/export support, likely using pglite.dataDirDump() and loadDataDir.