feat(dynamodb): partitionKey and sortKey are now immutable

[eladb]

Since partition key is a mandatory parameter for DynamoDB tables, by
our conventions it should be a required property and immutable.

BREAKING CHANGE: partitionKey is now a required property when defining a
dynamodb.Table. The addPartitionKey and addSortKey methods were removed.

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Pull Request Checklist

• Testing
  • Unit test added
  • 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
• Title and Description
  • Change type: title prefixed with fix, feat 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"
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  • IAM Policy Document (in @aws-cdk/aws-iam)
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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]

Thanks so much for taking the time to contribute to the AWS CDK ❤️

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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)
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• PR Description
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• Testing
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