Allow multiple provider specific annotations of same type

[roehrijn]

Description

External DNS already supports exposing a single K8s service behind multiple hostnames by using the external-dns.alpha.kubernetes.io/hostname annotation with a comma-seperated list of hostnames in its value. One would do this for example for making a service availible with a technical, static hostname and a DNS load-balancer, like Route53 can act as, at the same time.
However, when defining additonal annotations on the service, e.g. external-dns.alpha.kubernetes.io/aws-weight this always applies to all the hostnames of the service. This makes it impossible to define different weights for different hostnames on the same service. At the same time it is not possible that an annotation only applies to a single one and not all the hostnames of a service.

This PR implements a solution to make exactly the described configuration possible by introducing a kind of "string map" notation in the annotations' values.

For example:

external-dns.alpha.kubernetes.io/hostname: my.foo.app, my.foo.internal
external-dns.alpha.kubernetes.io/aws-weight: my.foo.app=100

would define the two hostname my.foo.app and my.foo.internal, whereas a load balancing weight is only set for my.foo.app.

Checklist

• Unit tests updated
• End user documentation updated

_{Jan Roehrich jan.roehrich@mercedes-benz.com, Mercedes-Benz Tech Innovation GmbH, legal info/Impressum}

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @roehrijn. Thanks for your PR.

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[mloiseleur]

/ok-to-test

[mloiseleur]

Thanks. It looks good, implementation is light and it's documented 👍
/lgtm
/assign @Raffo