feat(firestore): new generic pipeline expressions (#16550)

New Expressions:
- offset
- nor
- coalesce
- switch_on
- array_filter
- array_transform
- storage_size

Restructured how custom repr() strings are build, to accommodate
complexities in array_transform

Note: parent and reference_slice will be added in a future PR. More
detailed changes will be needed to support arbitrary collection/database
references; currently only document references are supported

Author: daniel-sanche