[KARAF-7317] Support spaces in boot features path

[jdmcmahan]

URI.create(String) throws a URISyntaxException if any of the resolved URIs from featuresRepositories contains a space.

This is common if customizing the boot features via the karaf-maven-plugin, then running the assembly from within a path containing spaces.

For example:

<plugin>
    <groupId>org.apache.karaf.tooling</groupId>
    <artifactId>karaf-maven-plugin</artifactId>
    <version>${karaf.version}</version>
    <configuration>
        <bootFeatures>
            <feature>standard</feature>
            <feature>my-feature</feature>
        </bootFeatures>
        ...
    </configuration>
</plugin>

will result in a new feature in etc as well as an entry in etc/org.apache.karaf.features.cfg:

featuresRepositories = file:${karaf.etc}/2b260482-45bc-427c-af65-8909c1f2f7a3.xml

However, if karaf.etc contains a space at runtime, Karaf fails to start because the URI cannot be parsed.

This PR simply replaces all spaces with %20 before passing the path to URI.create(String). The encodePath method can be further enhanced if other characters prove to be problematic.

[jdmcmahan]

I have created a companion ticket at KARAF-7317.