feat(s3): BucketEncryption Mixin for S3 server-side encryption

[mrgrain]

Reason for this change

The mixin codegen for specialized type-based mixins was living in mixins-preview, which is being phased out. This type of codegen needs to live in spec2cdk and generate output directly into aws-cdk-lib service modules.

The first mixin generated this way is BucketEncryption for S3, which provides a type-safe way to configure server-side encryption with full KMS key relationship resolution.

Description of changes

Introduces a new type-mixins codegen system in spec2cdk that generates specialized mixins from CloudFormation type definitions. Unlike the existing cfn-prop-mixins (which generate a mixin per resource with all properties), type-mixins focus on a single complex type and generate a dedicated mixin class with nested types, enums, and relationship-resolving flatten functions.

The codegen follows the same class-based pattern as L1PropsMixin and ResourceClass — a TypeMixin base class extends ClassType with a build() method. BucketEncryptionMixin is the first concrete implementation.

The generated BucketEncryption mixin delegates property assignment to the internal CfnPropsMixin with an override merge strategy, keeping the generated code thin while reusing existing infrastructure.

An isGeneratedMixinType check was added to awslint's CoreTypes to generically skip props-default-doc for types in the mixins namespace that come from .generated.ts files with @stability external. This avoids per-property exclusions in awslint.json.

Describe any new or updated permissions being added

N/A

Description of how you validated changes

9 unit tests covering BucketEncryption mixin: supports/rejects constructs, AES256, KMS with string key, KMS with IKeyRef, bucket key enabled, L2 .with(), enum values. All pass. Full yarn build of spec2cdk, awslint, and aws-cdk-lib passes with 0 errors.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.