This commit is a follow up on #124 which added the same type of
functionalities to the `helm/v3` package. Given all methods are
now supported by both version implementations, the generic client
interface has been updated to include the methods.

This was causing issues for airgapped or restricted instances of the
operator as adding a repository causes Helm to attempt to connect with
it, which may not be possible due to the earlier mentioned restrictions.

With this flag it is possible to import repositories from an index
file to a specific Helm client version.

Example:

`--helm-repository-import=v2:/var/v2.yaml,v3:/var/v3.yaml` causes
the operator to import the `v2.yaml` repositories in the Helm v2
client, and the `v3.yaml` repositories in the Helm v3 client.