Adds field shootAnnotations to flavors

[dguendisch]

What this PR does / why we need it:
Adds field annotations to flavors. It allows to add annotations to created shoot clusters (i.e. to trigger certain shoot behaviour).
Usage example:

flavors:
- provider: aws
  annotations:
    a: b
    c: d

Cooperates with gardener/gardener#2550

Release note:

Annotations for shoot clusters can be customized in a flavor by adding them to field `annotations`.

[schrodit]

any specific reason why you called it shootAnnotations in the shoot flavor instead of annotation or additionalAnnotation as I read all 3 cases in the code.
I think we should let the testrun annotation be shoot-annotations as it may clash.
But the flavor is only for shoots so I would rather go with annotation or addtionalAnnotation. WDYT?

[dguendisch]

Addressed and renamed the shoot flavor to addtionalAnnotation in the fields and just annotation in the flavor yaml.
ptal

[schrodit]

/lgtm