Use a map to cache the valueOf functions for enums

[osa1]

(This is cl/658131451 ported to open source, not my CL)

CL description: (slightly edited)

This is a generated-code size optimization.

The main change is to generate the valueOf functions for protobuf enums programatically, rather than (1) use the per-enum FooPbEnum.valueOf static function tear-foo which (2) uses a per-enum static variable that is lazily initialized to map from int value to the protobuf enum value.

With ~1000 protobuf enums, this allows most of the valueOf static function tear-off closures to tree-shaken, together with their functions and the code for the initializer of the map.

The secondary change is to use more specialized 'add' methods to BuilderInfo, allowing many fields to be 'defined' with a call with fewer arguments.

Together, these changes reduce the size of the main unit of an app by about 1.3%, and would have a similar effect on native targets.

This change requires dart-lang/sdk@175dc05, which was released with Dart 3.6.0.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:protobuf	5.0.0	(error) pubspec version (5.0.0) and changelog (5.0.0-wip) don't agree
package:protoc_plugin	23.0.0	ready to publish	protoc_plugin-v23.0.0

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.