Use RetentionPolicy CLASS in new 'ServiceContextKey' OSGi property type

[HannesWell]

And permit only types as target.

If the RetentionPolicy is only SOURCE Tycho does not recognize the annotation because the underlying BND library only operates on compiled classes and not on sources.

RetentionPolicy.CLASS is the default, but I still declared it to be specific in this case.

[github-actions]

Test Results

 1 758 files  ±0   1 758 suites  ±0   1h 33m 7s ⏱️ + 6m 32s
 4 170 tests ±0   4 148 ✅ ±0   22 💤 ±0  0 ❌ ±0 
13 107 runs  ±0  12 943 ✅ ±0  164 💤 ±0  0 ❌ ±0 

Results for commit f246856. ± Comparison against base commit e14565e.

[laeubi]

Probably PDE should warn if a property type is using source retention even though the spec only says

To be used in this way, the component property type must be annotated with the ComponentPropertyType meta-annotation so that, at build time, the annotation is recognized as a component property type.

there is really no benefit in using SOURCE retention type and it actually limits the usefulness with other tools (e.g. bnd/tycho).