refactor(core): misc cleanups to App-related APIs

[eladb]

Fixes #1891

Testing: all toolkit integration tests passed.

BREAKING CHANGE:

• cli: This release requires CDK CLI >= 0.34.0
• core: App.run() was renamed to App.synth() (soft deprecation, it will be removed in the next release).
• core: The Stack.autoDeploy feature has been removed. You can use cdk deploy STACK ... to determine which stacks to deploy (and wildcards are supported, so cdk deploy '*' will deploy all stacks. We plan to change the CLI to require specifying stacks if there is more than a single stack in the app (cdk deploy: require argument if there is more than a single stack #2750).
• core: ConstructNode.aspects is now private.
• core: The Synthesizer has been removed. Use ConstructNode.synth(node) instead.
• core: ISynthesizable.synthesize now accepts an ISynthesisSession which contains the CloudAssemblyBuilder object. This will allow further extension in the future.
• cx-api: cxapi.MissingContext now includes the context key
• core: Stack.reportMissingContext now accepts a single argument of type cxapi.MissingContext, which includes the missing context key
• core: Stack.annotatePhysicalName has been removed (not used).
• core: Stack.missingContext is now private.
• cx-api: Multiple changes to the cloud assembly APIs to reduce surface area and clean up.
• cdk-integ (private): if an integration test includes multiple stacks, use /// !cdk-integ STACK ... to explicitly specify which stacks to include in the test.

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• Decide if we want to rename app.run() to app.synth(). I think it will make it more clear that this is basically what "cdk synth" is invoking. Renamed!
• Add a @deprecated "App.run()" method so that I won't get stoned.

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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

[mergify]

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checklist:

• PR title type(scope): text
  • type: fix, feat, refactor go into CHANGELOG, chore is hidden
  • scope: name of module without aws- or cdk- prefix or postfix (e.g. s3 instead of aws-s3-deployment)
  • text: use all lower-case, do not end with a period, do not include issue refs
• PR Description
  • Rationale: describe rationale of change and approach taken
  • Issues: indicate issues fixed via: fixes #xxx or closes #xxx
  • Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>
• Testing
  • Unit test added. Prefer to add a new test rather than modify existing tests
  • CLI or init templates change? Re-run/add CLI integration tests
• Documentation
  • README: update module README to describe new features
  • API docs: public APIs must be documented. Copy from official AWS docs when possible
  • Design: for significant features, follow design process