Add NodeValidationVisitor feature that enforces base64 encoding of blob values

[brandondahler]

Background

• What do these changes do?
  • Adds a new REQUIRE_BASE_64_BLOB_VALUES NodeValidationVisitor Feature which enforces that blob values are valid base64-encoded values
  • Fixes BlobLengthPlugin to enforce length on the base64-decooded byte array's length instead of the base64-encoded string's length when the new feature is enabled
• Why are they important?
  • Per both the 1.0 and 2.0 specs for trait node values, blob values are "A string value that is base64 encoded."; however, existing trait validators and related code generators do not necessarily follow this specification.
    • Some trait validators like the DefaultTraitValidator only warn on invalid values
    • Some codegen implementations like smithy-typescript's HttpProtocolTestGenerator explicitly parse the blob value as an array of utf-16 bytes

Testing

• How did you test these changes?
  • Left all existing tests alone
  • Added new parameterized test to verify both positive and negative cases for the blob behavior when the feature is enabled

Links

• Links to additional context, if necessary
  • Mentioned this feature in the comments of Feature request: Support for defining and verifying positive and negative test vectors for constrained shapes and members #2821
• Issue #, if applicable (see here for a list of keywords to use for linking issues)
  • No direct issue created

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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.

[JordonPhillips]

Great work! Once this is in I'll need to add it to the new event stream protocol test trait