XML Reader and Reader2Doc Improvements

[DevinTDHa]

Description

This PR changes how Reader2Doc behaves, especially for XML files:

1. Reader2Doc will now create single document annotations per default. This should result in a more expected behavior when reading large documents. By default, lines are joined by the newline character \n but can be set with setJoinString.
2. XML reader now ignores empty tags without text content. To extract attributes of tags, the new flag setExtractTagAttributes(attributes: list[str]) will trigger addition of the values to the document output. For example, for the test.xml

<bookstore>
    <book category="children">
        <title lang="en">Harry Potter</title>
        <author>J K. Rowling</author>
        <year>2005</year>
        <price>29.99</price>
    </book>
    <book category="web">
        <title lang="en">Learning XML</title>
        <author>Erik T. Ray</author>
        <year>2003</year>
        <price>39.95</price>
    </book>
</bookstore>

We can extract category and lang values with the Reader2Doc Config

reader2doc = Reader2Doc() \
    .setContentType("application/xml") \
    .setContentPath("../src/test/resources/reader/xml/test.xml") \
    .setOutputCol("document") \
    .setExtractTagAttributes(["category", "lang"])

Resulting in

children
en
Harry Potter
J K. Rowling
2005
29.99
web
en
Learning XML
Erik T. Ray
2003
39.95