Add attributes to Invitation

[xgp]

Some invitation use cases need a way of passing/persisting additional context metadata within the invite, such as additional properties about the user-to-be-invited. The model currently supports a roles[] array; but could benefit a free-form key-value Map<String, String> attributes map (much like many of the other Keycloak models), so that additional context can be passed to the consumer.

In scope:

1. Add an invitation attributes table to the migrations
2. Add attributes to the model and representation
3. Add a new method GET /:realm/orgs/:orgId/invitations/:inviteId to get invitations by id
4. Make the attributes map available on the FTL context for the invitation email

Out of scope:

1. Transferring those attributes after the invitation is accepted/denied
2. Building Admin UI or Admin Portal UI to allow setting and/or viewing the attributes (maybe in the future)

[xgp]

cc @phamann

[xgp]

fixed in #153