Werk #14067: REST-API host_config: addition of validation for host labels

Host labels take the key:value format where both key and value are of the string type. In the previous version, it was possible to specify non valid entries such as a list of strings as the REST API did not perform validation for the labels. This consequently led to other problems as Checkmk attempted to process the label values expecting a string. This werk introduces validation for the host labels in order to prevent such instances.

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