Support spaces in boot features path

`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:

```xml
<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.

I have proposed a solution with this PR.