chore(cli): convert the bootstrap template from JSON to YAML (aws#6799)

* chore(cli): convert the bootstrap template from JSON to YAML.

This allows us to add comments to the template,
which make it clearer why certain things
(most noticeably, permissions)
are required.

While I was in the area,
fixed a small issue with the bootstrap integ test
(the test app wasn't respecting the env the bootstrap stack was created in).

* Reuse YAML parsing routines we already have

Co-authored-by: Rico Huijbers <rix0rrr@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

design/cdk-bootstrap.md

@@ -323,4 +323,4 @@ This should make sure the CFN update succeeds.
 ## Bootstrap template
 
 The bootstrap template used by the CLI command can be found in the
-[aws-cdk package](../packages/aws-cdk/lib/api/bootstrap/bootstrap-template.json).
+[aws-cdk package](../packages/aws-cdk/lib/api/bootstrap/bootstrap-template.yaml).

packages/aws-cdk/lib/api/bootstrap/bootstrap-environment2.ts

@@ -3,6 +3,7 @@ import * as fs from 'fs-extra';
 import * as os from 'os';
 import * as path from 'path';
 import { BootstrapEnvironmentProps, deployStack, DeployStackResult } from '..';
+import { loadStructuredFile } from '../../serialize';
 import { SdkProvider } from '../aws-auth';
 
 export async function bootstrapEnvironment2(environment: cxapi.Environment, sdk: SdkProvider,
@@ -14,11 +15,14 @@ export async function bootstrapEnvironment2(environment: cxapi.Environment, sdk:
 
   const outdir = await fs.mkdtemp(path.join(os.tmpdir(), 'cdk-bootstrap-new'));
   const builder = new cxapi.CloudAssemblyBuilder(outdir);
-  const templateFile = `${toolkitStackName}.template.json`;
 
-  await fs.copy(
-    path.join(__dirname, 'bootstrap-template.json'),
-    path.join(builder.outdir, templateFile));
+  // convert from YAML to JSON (which the Cloud Assembly uses)
+  const templateFile = `${toolkitStackName}.template.json`;
+  const bootstrapTemplatePath = path.join(__dirname, 'bootstrap-template.yaml');
+  const bootstrapTemplateObject = loadStructuredFile(bootstrapTemplatePath);
+  await fs.writeJson(
+    path.join(builder.outdir, templateFile),
+    bootstrapTemplateObject);
 
   builder.addArtifact(toolkitStackName, {
     type: cxapi.ArtifactType.AWS_CLOUDFORMATION_STACK,