[Madrigal]

Issue #, if available: #3891

Description of changes:

XML request payloads (S3, S3 CRT, S3 Control, CloudFront, Route 53) were serialized with XmlDocument::ConvertToString(), which uses tinyxml2's default pretty-printing. That adds indentation and newlines to every element. For a CompleteMultipartUpload with 9,331 parts the body is 1,090,743 bytes (1.04 MiB). Some S3-compatible services with a 1 MiB request/XML limit, such as Ceph RGW, reject it even though the upload is under the 10,000-part maximum.

Changes:

• Add XmlDocument::ConvertToCompactString(), which prints the same document without whitespace between elements. Whitespace inside text content is unchanged.
• ConvertToString() is unchanged and still pretty-prints, since it is public API and may be used outside the SDK, e.g. to save a document to a file. Both methods' doc comments now point to each other.
• Switch the codegen templates (XmlRequestSource.vm, S3 PutBucketNotificationConfigurationRequest.vm) to ConvertToCompactString(), and regenerate the affected clients. That's 190 files, each a one-line change: payloadDoc.ConvertToString() → payloadDoc.ConvertToCompactString().
• Add unit tests for compact output: exact bytes, nested elements, escaping, newlines and spaces in text surviving a round trip through the parser, and empty output for a moved-from document.

Result: the 9,331-part CompleteMultipartUpload body shrinks to 829,472 bytes (0.79 MiB), and 10,000 parts comes to 0.85 MiB.

This covers S3Client and S3CrtClient::CompleteMultipartUpload, and Transfer Manager v1. Automatic multipart uploads through S3CrtClient::PutObject and the S3 Transfer Manager v2 build this body inside aws-c-s3, which hard-codes the same whitespace. That needs a separate fix in awslabs/aws-c-s3.

This also replaces the approach in #3892 (a ClientConfiguration flag), which was closed in review in favor of not threading serializer options through requests.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change. The API change is additive only. Request bodies change byte-for-byte but stay
  semantically identical XML.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

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