feat(cli): add --build option (aws#19663)

Adds a `--build` option to the CDK CLI so that customers can specify pre-synth build commands without modifying their cdk.json settings. Customers can use this feature to run special build commands during `cdk watch` that cdk should not run during a `cdk synth`.

Fixes aws#19667

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `cdk-integ` to deploy the infrastructure and generate the snapshot (i.e. `cdk-integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk/README.md

@@ -643,7 +643,7 @@ Some of the interesting keys that can be used in the JSON configuration files:
 ```json5
 {
     "app": "node bin/main.js",        // Command to start the CDK app      (--app='node bin/main.js')
-    "build": "mvn package",           // Specify pre-synth build           (no command line option)
+    "build": "mvn package",           // Specify pre-synth build           (--build='mvn package')
     "context": {                      // Context entries                   (--context=key=value)
         "key": "value"
     },

packages/aws-cdk/lib/cli.ts

@@ -56,6 +56,7 @@ async function parseCommandLineArguments() {
     .env('CDK')
     .usage('Usage: cdk -a <cdk-app> COMMAND')
     .option('app', { type: 'string', alias: 'a', desc: 'REQUIRED: command-line for executing your app or a cloud assembly directory (e.g. "node bin/my-app.js")', requiresArg: true })
+    .option('build', { type: 'string', desc: 'Command-line for a pre-synth build' })
     .option('context', { type: 'array', alias: 'c', desc: 'Add contextual string parameter (KEY=VALUE)', nargs: 1, requiresArg: true })
     .option('plugin', { type: 'array', alias: 'p', desc: 'Name or path of a node package that extend the CDK features. Can be specified multiple times', nargs: 1 })
     .option('trace', { type: 'boolean', desc: 'Print trace for stack warnings' })

packages/aws-cdk/lib/settings.ts

@@ -265,6 +265,7 @@ export class Settings {
     return new Settings({
       app: argv.app,
       browser: argv.browser,
+      build: argv.build,
       context,
       debug: argv.debug,
       tags,

packages/aws-cdk/test/settings.test.ts

@@ -144,3 +144,14 @@ test('should include outputs-file in settings', () => {
   // THEN
   expect(settings.get(['outputsFile'])).toEqual('my-outputs-file.json');
 });
+
+test('providing a build arg', () => {
+  // GIVEN
+  const settings = Settings.fromCommandLineArguments({
+    _: [Command.SYNTH],
+    build: 'mvn package',
+  });
+
+  // THEN
+  expect(settings.get(['build'])).toEqual('mvn package');
+});