[keycloak-dev] Import proposal

Currently we support importing a complete realm definition using the import/export feature. Issues with the current approach is: * Only complete realm - not possible to add to an existing realm * No good feedback if import was successful or not * Use of system properties to initiate the import is not very user friendly * Not very elegant for provisioning. For example a Docker image that want's to bundle some initial setup ends up always running the import of a realm, which is skipped if realm exists To solve this I've come up with the following proposal: Allow dropping representations to be imported into 'standalone/import'. This should support creating a new realm as well as importing into an existing realm. When importing into an existing realm we will have an import strategy that is used to configure what happens if a resource exists (user, role, identity provider, user federtation provider). The import strategies are: * Skip - existing resources are skipped, * Fail - if any resource exists nothing is imported * Overwrite - any existing resources are deleted. The directory will be scanned at startup, but there will also be an option to monitor this directory at runtime. To prevent a file being imported multiple times (also to make sure only one node in a cluster imports) we will have a table in the database that contains what files was imported, from what node, date and result (including a list of what resources where imported, which was not, and stack trace if applicable). The primary key will be the checksum of the file. We will also add marker files (<json file>.imported or <json file>.failed). The contents of the marker files will be a json object with date imported, outcome (including stack trace if applicable) as well as a complete list of what resources was successfully imported, what where not. The files will also allow resolving system properties and environment variables. For example: { "secret": "${env.MYCLIENT_SECRET}" } This will be very convenient for example with Docker as it would be very easy to create a Docker image that extends ours to add a few clients and users. It will also be convenient for examples as it will make it possible to add the required clients and users to an existing realm.