bootstrap a dart backend for sidekick

[devoncarew]

This is a meta issue to track bootstrapping a Dart backend for sidekick (this is intended to track initial, early work on a generator).

• land a hello-world generator (generating mostly empty files)
• the main library file for package:google_client_api should be called api.dart (not client.dart)
• emit enums
• handle emitting primitive types to dart types (string, int32, int64, ...)
• emit messages
• create a WKT for google.protobuf.FieldMask
• create a WKT for google.protobuf.Duration
• add basic tests for the implementation
• emit service method calls
• dart format generated source
• escape dart keywords
• update proto doc references; [SecretVersion][google.cloud.secretmanager.v1.SecretVersion] ==> [SecretVersion]
• determine the most idoimatic way to namespace nested messages and enums (i.e., SecretVersion$State vs SecretVersionState vs SecretVersion_State).
• create a WKT for google.protobuf.Timestamp
• create a WKT for google.protobuf.Any
• handle BYTES_TYPE (BytesValue?)
• generate google/cloud/location
• generate google/iam

[devoncarew]

Longer term, we'll also want to decide whether we're generating one library file per API or one file per service (or per backing proto definition file?).

package:kms
  lib/
    kms.dart (w/ all n services)

vs.

package:kms
  lib/
    kms.dart (mostly empty, importing and exporting the two libs in src)
    src/
      kms.dart (w/ KeyManagementService )
      kms_service.dart (w/ EkmService)