Expose some modified MagmaCoreDatabase methods on MagmaCoreService

[twalmsley]

A project I'm working on needs two additional methods as shown in the code below, with some slight impact on MagmaCoreDatabase and its implementers.

1. The first method is useful for finding all items with a certain predicate, such as HQDM.ENTITY_NAME, HQDM.MEMBER_OF_KIND, or RDFS.RDF_TYPE
2. The second method is a more specific version on the first for finding objects of a specific RDFS.RDF_TYPE or members of a specific kind etc.

    /**
     * Find objects by a predicate.
     *
     * @param <T>       HQDM entity type.
     * @param predicate the predicate {@link HqdmIri}
     * @return a List of {@link Thing} that were found.
     */
    public <T extends Thing> List<T> findByPredicateIriOnly(final IRI predicate) {
        final List<T> searchResult = (List<T>) database.findByPredicateIriOnly(predicate);
        return searchResult;
    }

    /**
     * Find objects by a predicate value.
     *
     * @param <T>       HQDM entity type.
     * @param predicate the predicate {@link IRI}
     * @param value     The value of the predicate.
     * @return a List of {@link Thing} that were found.
     */
    public <T extends Thing> List<T> findByPredicateIriAndValue(final IRI predicate, final Object value) {
        final List<T> searchResult = (List<T>) database.findByPredicateIriAndValue(predicate, value);
        return searchResult;
    }

[twalmsley]

Waiting on som PRs so I can rebase this change and create another PR.