Protocol codegen fixes via protocol tests

[kstich]

Contains the following fixes, each in their own commit for ease of review:

• Fix timestamp serde around TS string types

const labelValue: string = Math.round(input.memberEpochSeconds.getTime() / 1000).toString();

• Update parseFloat call to remove second param

contents.headerDouble = parseFloat(output.headers['x-double']);

• Add metadata to modeled error outputs

response = {
    ...await deserializeAws_restJson1_1ComplexErrorResponse(output, context),
    $metadata: deserializeMetadata(output),
}

• Update output shapes being MetadataBearers
  This commit removes the MetadataBearer type for non-error output
  shapes in the model. This makes them not require having a metadata
  property when being reused as non-response shapes (batches, etc.)
  Output shapes instead become MetadataBearers as part of compound
  types in the Command and Client codegen.

// Command specific alias shape is also the MetadataBearer
export type RecursiveShapesCommandOutput = RecursiveShapesInputOutput & __MetadataBearer;

// Client output union uses output MetadataBearers as well
export type ServiceOutputTypes =
  | __MetadataBearer
//  ...
  | RecursiveShapesInputOutput & __MetadataBearer;

• Fix handling for query collection deserialization
  This commit updates query collection serialization to properly create
  Set types. It also fixes an issue where query collections of non-string
  simple types were not deserializing to their proper primitive type.

// Handle query collections of non-strings
contents.headerBooleanList = (output.headers['x-booleanlist'] || "").split(',').map(_entry => _entry === 'true');

// And handle sets
contents.headerStringSet = new Set((output.headers['x-stringset'] || "").split(',').map(_entry => _entry));

• Fix header handling in pre-parsed errors

if (parsedOutput.headers["x-header"] !== undefined) {
    contents.Header = parsedOutput.headers['x-header'];
  }

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