V2: Make DescField.presence available for all fields

[timostamm]

The property DescField.presence tells whether a field tracks presence, but only for singular fields. This change makes it available for all fields. The values match with the documentation, which should be helpful for users who want to take a closer look at the schema.

The property is also available for extension descriptors. It's not well documented, but implicit presence (e.g. a proto3 singular scalar field) does not apply to extensions. Otherwise, setting an extension value 0 would be ambigous.

[timostamm]

Adding coverage because the logic for deciding when to emit changed.

[timostamm]

The function canEmitFieldDefaultValue is replaced by this simple check, matching the specified behavior:

Fields that don’t support presence and that have their default value are omitted by default in JSON output (for example, an implicit presence integer with a 0 value, implicit presence string fields that are empty strings, and empty repeated and map fields).

We might want to rename the serialization option emitDefaultValues to emitFieldsWithoutPresence as a follow-up.

[timostamm]

@srikrsna-buf, the presence and other properties of DescField/DescExtension are computed eagerly (that wasn't the case some time ago), and I can't think of a way that the kludge we use in the extension container should cause any more problems. So I think we should keep the extension container.