- add `isNotDefault` extension methods for core proto types

This is the hand-written portions of
#58. I'll follow
this up w/ generator changes (in the librarian repo), and then a PR back
to this repo that will re-generate the packages and update test and
example code (the rest of #58).

Happy to bike-shed on the extension method name(s). It's `isNotDefault`
now, but could be something like `notDefault`, ... .

cc @natebosch

Use proto presence info to generate null-safe code:

- `annotateField` updated to use information from proto optional
annotations
- toJson() implementations write values if they're nullable and have a
non-null value, or if they're not nullable and have a non-default value
for that type
- fromJson parses a value for parse a value for nullable fields if one
is given; it will parse any value for non-null field or fall back to
using the default value for that type
- properly dereference null values when referring to request fields for
use in url paths
- enums definitions now get a default value (`EnumClass.$default`)
- removed the older annotate.requiredFields and tests (based on usage in
service calls)
- closes googleapis/google-cloud-dart#24

Tests are updated for `buildQueryLines()` to include optional fields.
Tests were added for the `createFromJsonLine()` and `createToJsonLine()`
methods; previously we got our testing here from being on the same repo
as the dart code, and running the dart unit tests on the generator's
putput.

In-lined from the PR:

>	- proto 3 fields default to implicit presence
>	- the 'optional' keyword changes a field to explicit presence
>	- types like lists (repeated) and maps are always implicit presence
>	
> Explicit presence means that you can know whether the user set a value
or
> not. Implicit presence means you can always retrieve a value; if one
had
>	not been set, you'll see the default value for that type.
>	
> We translate explicit presence (a optional annotation) to using a
nullable
> type for that field. We translate implicit presence (always returning
some
>	value) to a non-null type.
>	
>	Some short-hand:
>	  - optional == explicit == nullable
>	  - implicit == non-nullable
>	  - lists and maps == implicit == non-nullable
>	  - singular message == explicit == nullable

Sample output can be seen at
googleapis/google-cloud-dart#58.

Update the packages here using the null-safety generator change:

- a follow up to googleapis/librarian#2726
- update sidekick version reference
- re-generate the code
- update the hand-written code (part files, examples, and tests)

Previously as a draft at
#58; done as
separate commits to make review a little easier.

@brianquinlan, this does not include sidekick / dart package version
changes. I assume we'll want to bump everything (to 0.2.0?). Not sure if
we're bumping on changes or on publish (or don't have a specific process
yet).