More conventional conversion yaml to xml

[vitlibar]

Changelog category:

• Improvement

Changelog entry:

Make conversion YAML -> XML more conventional. A sequence of mappings like

seq:
  - k1: val1
    k2: val2
  - k3: val3

is now converted into the following xml:

<seq><k1>val1</k1><k2>val2</k2></seq>
<seq><k3>val3</k3></seq>

For example,

remote_servers:
    test_cluster:
        shard:
          - replica:
                host: 127.0.0.1
                port: 9000
          - replica:
                host: 127.0.0.2
                port: 9000

is now parsed by YAMLParser as

<remote_servers>
    <test_cluster>
        <shard>
            <replica>
                <host>127.0.0.1</host>
                <port>9000</port>
            </replica>
        </shard>
        <shard>
            <replica>
                <host>127.0.0.2</host>
                <port>9000</port>
            </replica>
        </shard>
    </test_cluster>
</remote_servers>

This PR fixes #40561 and reverts #35135

[Avogar]

AST fuzzer (ubsan) - #29039
Stress test (ubsan) - #41013