DATAREST-230 - Support for serving Alps documents.

This commit adds the support to expose additional resources that serve ALPS [0] resources to document the available state transitions and representations. The exposure is enabled by default and can be customized using the RepositoryRestConfiguration.metadataConfiguration() object.

Currently the set of descriptors exposed includes:

- A descriptor for the representation of the domain type. Linkable associations are represented as safe descriptors, ones that are not linked are semantic descriptors.
- Descriptors for each supported HTTP method for both the collection and item resources to indicate the ability to update, create, delete etc.
- Safe descriptors (e.g. to access the collection or item resource) get potentially available customizations (pagination, projections) attached through nested descriptors.

Documentation

An ALPS descriptor contains a doc attribute to carry semantic information for the end user or a potential client to display. The information can be described in two ways: the first one is the @Description annotation that captures the plain text information one wants to get listed. It is supported in @RepositoryRestResource, on query methods, projection interfaces and accessors etc.

The preferred approach however is to use a resource bundle rest-messages.properties. For each descriptor we will resolve a key starting with rest.description followed by a dot path into the resource. By default, doc attributes are only rendered if the resource bundle contains an entry for the relevant key. You can enforce displaying unresolved keys by configuring MetadataConfiguration.omitUnresolvableDescriptionKeys(…).

For representation descriptors and and the parameter list of query method descriptors we will display enum values by default as comma-separated list. The list is also available as message resolution argument, so that you can refer to the list in your description message via the {0} placeholder.

TODOs:

- Improve descriptors for associations (indicate ability to update etc.)

[0] ALPS - alps.io