Add apiserver/actions ServiceCharmActions endpoint

Also adds supporting types in apiserver/params/params.go and
apiserver/params/actions.go.
Add apiserver/actions ServiceCharmActions endpoint

Applied naming changes per @TheMue.
Merge pull request #909 from binary132/apiserver-servicecharmactions

Add apiserver/actions ServiceCharmActions endpoint

Also adds supporting types in apiserver/params/params.go and
apiserver/params/actions.go.