feat!: Replace the XML encoder with a custom Smithy implementation

[jbelkins]

Issue #

awslabs/aws-sdk-swift#964

(this is the XML encoder part of the project. This is free-standing and can merge by itself without the decoder.)

Note that only the RestXML Smithy protocol uses XML encoding, the other protocols either use JSON or form URL encoding for their requests. So this new encoder affects only about six services that use RestXML as their Smithy protocol.)

The third-party XMLCoder library remains in the project because it is still used for decoding. The PR for the new XML decoder will follow once this PR merges, and will be built with similar patterns. At that time, the XMLCoder library will be removed once and for all.

Description of changes

Eliminates the use of the third-party XMLCoder library for encoding XML, replacing it with a first-party XML encoding solution. As a result, the codegen for XML encoding is greatly simplified, and the generated code for these types is much more compact.

• A new SmithyReadWrite public library is added to this project, with standard interfaces for serialization based on the use of modular closures.
• A new SmithyXML public library is added to this project, which contains the new XML encoder.
• A new SmithyTimestamps non-public library is added, which allows ClientRuntime and SmithyXML to share the timestamp-related code.
• On Linux systems, libxml2 is packaged as a Swift module for use by SmithyXML. (Apple platforms come with libxml2 already modularized and do not need this module.)
• Encodable conformance for restXML model types is replaced by per-type static methods that perform the encoding. This allows for hiding the encoding implementation from the model's public interface, and for static dispatch during encoding for greater performance. Type-safety is still achieved through the use of generics where needed.
• Because Foundation's best tool for handling XML (the XMLDocument class) is not available on Apple platforms other than Mac, libxml2 is used directly to parse XML. libxml2 is available standard on all Apple platforms, and is part of the standard Swift install for Linux, so no special dependency management should be required.
• Code generation has been simplified greatly by moving much of what was previously code-generated encoding logic into SmithyXML.

Other changes:

• Universal encoding interfaces are introduced in the middleware that performs encoding of both regular requests and event streams. The JSON and Form URL encoders that are currently based on Swift Codable are adapted to this interface as well.
• The HTMLBodyMiddleware that was previously code generated with complex alternate code paths has been split into several single purpose middlewares in ClientRuntime that can be used as needed.
• Replaces the Box<T> class that was previously used to hold self-referencing properties with the @Indirect Swift property wrapper, which holds the member in a reference like Box<T> but requires less custom codegen to implement.

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[indirect]

hi, I am a person, and when you type @indirect you subscribe me to your tickets. you may not want to do that in the future, unless you are actually looking for feedback from me. 😅