Adapt to XML API changes

[graebm]

Issue
The XML API was hard to use right, leading to bugs like this: #328

Description of changes:

• Adapt to API changes from: Simplify error handling in XML API - BREAKING CHANGE aws-c-common#1043
• Break up node traversal functions, to ensure we're processing the correct XML elements.
  • Previously, the same callback would be used for all XML elements. This could cause error if an element with the same name occurred at different parts of the document tree.
• Improved error checking
  • Previously, many calls to aws_xml_node_as_body() weren't being checked for error.
• Replace aws_xml_get_top_level_tag() and aws_xml_get_top_level_tag_with_root_name() with aws_xml_get_body_at_path()
  • aws_xml_get_top_level_tag() didn't check the name of the root node
  • aws_xml_get_top_level_tag_with_root_name() was clunky to use (IMHO)
  • so replace with an API that can retrieve an element at any depth (not just 2), checking names the whole way, and with a nicer API (IMHO).
  • new function gives aws_byte_cursor instead of aws_string, the user was usually just deleting it afterwards, which made their error-handling more complicated.
• Trivial stuff:
  • Remove unused functions aws_s3_list_objects_operation_new() and aws_s3_initiate_list_parts()
  • aws_replace_quote_entities() returns aws_byte_buf by value, instead of as out-param
  • Some functions take aws_byte_cursor by value, instead taking aws_string * or aws_byte_buf * or aws_byte_cursor * by pointer

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

[codecov-commenter]

Codecov Report

Merging #332 (c08cd14) into main (bc9c8b2) will increase coverage by 0.48%.
The diff coverage is 93.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
+ Coverage   88.93%   89.42%   +0.48%     
==========================================
  Files          17       17              
  Lines        4972     4888      -84     
==========================================
- Hits         4422     4371      -51     
+ Misses        550      517      -33     

Impacted Files	Coverage Δ
source/s3.c	96.15% <ø> (ø)
source/s3_paginator.c	88.23% <80.76%> (-2.50%)	⬇️
source/s3_list_parts.c	92.70% <88.23%> (+17.89%)	⬆️
source/s3_auto_ranged_put.c	92.19% <88.88%> (-0.25%)	⬇️
source/s3_auto_ranged_get.c	98.55% <100.00%> (-0.01%)	⬇️
source/s3_copy_object.c	81.14% <100.00%> (-0.13%)	⬇️
source/s3_list_objects.c	92.38% <100.00%> (+12.07%)	⬆️
source/s3_meta_request.c	92.73% <100.00%> (-0.61%)	⬇️
source/s3_util.c	98.66% <100.00%> (-0.07%)	⬇️