add tycho maven build support for bnd

This adds support building bnd with Eclipse Tycho 4.0.13
See https://tycho.eclipseprojects.io/doc/4.0.13/BndBuild.html

e.g. build inside the repo root with:
`mvn clean verify`

Signed-off-by: Christoph Rueger <chrisrueger@gmail.com>

Author: chrisrueger

.mvn/extensions.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<extensions>
+   <extension>
+      <groupId>org.eclipse.tycho</groupId>
+      <artifactId>tycho-build</artifactId>
+      <version>${tycho-version}</version>
+   </extension>
+</extensions>

.mvn/maven.config

@@ -0,0 +1 @@
+-Dtycho-version=4.0.13