[KARAF-6017] - added cache facade

[awrb]

Draft phase (it works but it's not a proper component yet).

I'm opening a pr already so it can be reviewed before putting more work into it.

Some features:

• it uses ehcache with JSR 107 API
• the api does not depend on ehcache, so other providers could be added
• possible to load ehcache configuration from xml
• some karaf commands for debugging/playing around with the caches

Some open points:

• how should the interface (CacheService) look like
• is anything important not possible because of the facade (see the docs: https://www.ehcache.org/documentation/3.0/index.html)

[awrb]

@jbonofre would be great if you can take a look :smiley: once we agree on the requirements then I'll start adding javadocs, tests, feature etc.

[awrb]

@jbonofre I made a proper pull request now (added docs, example, tests etc). I'm happy to introduce any changes if needed.