[pkg/ottl] Add AddElementXML Converter

[djaglowski]

This simple converter allows a basic form of XML document modification. Aside from the obvious, this can be used to disambiguate elements which should be members of a slice when parsed by the proposed ParseSimplifiedXML.

To illustrate this, consider how you would expect the following two documents to parse:

<Files>
   <File>foo.txt</File>
</Files>

<Files>
   <File>foo.txt</File>
   <File>bar.txt</File>
</Files>

The second document can clearly be understood as a slice of File elements:

Files:
  - File: foo.txt

but the first document could be misunderstood as a map where File is a unique key:

Files:
  File: foo.txt

That is, there is no way to infer the schema from a single example. However, by adding an empty File element, we can disambiguate the schema.

<Files>
   <File>foo.txt</File>
   <File/>
</Files>

Many users will not care about this detail but for those who need the schema to be interpreted consistently, this mechanism provides a solution. Note that in the proposed ParseSimplifyXML converter, empty elements without text content are dropped, since they would be keys without values in a map.

[djaglowski]

Closed in favor of #35436