Make the pdf decoration pluggable on the signature plugin

Extend extension point signature of signature service that allows to contribute a SignatureAppearanceFactory implementation in order to customize the signature appearance inserted in the signed PDF.
Right now, the message inserted by the signature (This document has already been signed by) is hard-coded in the service and the information mentioned (user signing the document and the date) cannot be changed.
The SignatureAppearanceFactory interface will define a method public void format(PdfSignatureAppearance pdfSignatureAppearance, DocumentModel doc, String principal, String reason).
A default implementation will be provided with the current signature appearance defined in the signature service (see SignatureService.signPDF()).
example of contribution:

<extension point="signature" target="org.nuxeo.ecm.platform.signature.api.sign.SignatureService">
  <configuration>
    <reason>This document signed as an example.</reason>
    <layout columns="3" id="defaultConfig" lines="5" startColumn="1" startLine="1" textSize="10"/>
    <appearanceFactory class="org.nuxeo.ecm.platform.signature.core.sign.DefaultSignatureAppearanceFactory" />
  </configuration>
</extension>