P2Export: support update site url per feature

[chrisrueger]

Closes #6788

Now you can override the update URL per feature and it is now also added to feature.xml and content.xml This makes it easier to install features which require a specific update site to be available in Eclipse without the user having to add the site manually.

e.g.

#bndtools.ecf.feature.bndrun

Bundle-Name:            Bndtools ECF Remote Services
Bundle-Description:     ECF Remote Services integration for Bndtools
Bundle-Category         bndtools
Bundle-Copyright: Copyright Composent, Inc. others, 2025
update: https://download.eclipse.org/rt/ecf/latest/site.p2/
update.label: ECF Update Site

Require-Capability \
    feature;name=org.eclipse.ecf.core.feature;version="[0.0.0,1000)",\
    feature;name=bndtools.main.feature,\
    feature;name=org.eclipse.ecf.discovery.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.discovery.jmdns.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.osgi.services.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.provider.generic.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.provider.generic.remoteservice.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.remoteservice.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.remoteservice.sdk.bndtools.feature;version="[0.0.0,1000)",\
    feature;name=org.eclipse.ecf.sharedobject.feature;version="[0.0.0,1000)"

results in

e.g. feature.xml

 <url>
    <update url="https://download.eclipse.org/rt/ecf/latest/site.p2/" label="ECF Update Site"/>
  </url>

and also to content.xml <references> e.g.

<references size="4">
    <repository uri="https://bndtools.jfrog.io/bndtools/update-latest" url="https://bndtools.jfrog.io/bndtools/update-latest" type="1" options="0"/>
    <repository uri="https://bndtools.jfrog.io/bndtools/update-latest" url="https://bndtools.jfrog.io/bndtools/update-latest" type="0" options="0"/>
    <repository uri="https://download.eclipse.org/rt/ecf/latest/site.p2/" url="https://download.eclipse.org/rt/ecf/latest/site.p2/" type="1" options="1"/>
    <repository uri="https://download.eclipse.org/rt/ecf/latest/site.p2/" url="https://download.eclipse.org/rt/ecf/latest/site.p2/" type="0" options="1"/>
  </references>