Implemented EZP-21245: Make it easier to register eZXML custom tags in main XSL stylesheet

This PR adds the possibility to dynamically load custom XSL stylesheets for XMLText rendering (e.g. for *custom tags*).

## Example
Implementation for a `youtube` custom tag (see [jvEmbedTag legacy implementation](https://github.com/lolautruche/jvEmbedTag)).

`ezpubish.yml`:
```yaml
ezpublish:
    system:
        my_siteaccess_group:
            fieldtypes:
                ezxml:
                    custom_tags:
                        - { path: %kernel.root_dir%/../src/Acme/TestBundle/Resources/embedTag.xsl, priority: 10 }
```

XSL stylesheet:
```xslt
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet
        version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:xhtml="http://ez.no/namespaces/ezpublish3/xhtml/"
        xmlns:custom="http://ez.no/namespaces/ezpublish3/custom/"
        xmlns:image="http://ez.no/namespaces/ezpublish3/image/"
        exclude-result-prefixes="xhtml custom image">

    <xsl:output method="html" indent="yes" encoding="UTF-8"/>

    <xsl:template match="custom[@name='youtube']">
        <div class="jvembed jvembed-youtube">
            <iframe>
                <xsl:attribute name="width"><xsl:value-of select="@custom:videoWidth"/></xsl:attribute>
                <xsl:attribute name="height"><xsl:value-of select="@custom:videoHeight"/></xsl:attribute>
                <xsl:attribute name="src"><xsl:value-of select="@custom:video"/></xsl:attribute>
                <xsl:attribute name="frameborder">0</xsl:attribute>
                <xsl:attribute name="allowfullscreen"/>
            </iframe>
        </div>
    </xsl:template>

</xsl:stylesheet>
```

## Tasks

* [x] Implement the base mechanism
* [x] Add semantic configuration