Werk #16016: mk_postgres: addition of option to specify instance name

The migration of the postgres_instances check plugin to the new CheckAPI broke the discovery mechanism of the plugin as it previously allowed an instance to have no specified name. This werk introduces the option to specify an explicit name for an instance. The mk_postgres plugin is adjusted accordingly and raises an error if the name cannot be inferred from .env file. To benefit from this change and also resolve the error, the user should adjust or bake the config file again and roll out the newly adapted plugin.

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