Implement ols api

[RalphBln]

This PR advances the implementation of the API gateway's OLS V1 and V2 (aka OLS3 and OLS4) API's, so that legacy applications formerly developed against OLS can seamlessly communicate with the API gateway.

The current state of the implementation is as follows:

V1 (OLS 3)

• /search
• /select
• /suggest
• /terms (Depends on #123)
• /terms?iri=https://xxx
• /ontologies/{onto}/terms
• /ontologies/{onto}/terms?iri=https://xxx
• /ontologies/{onto}
• /ontologies

V2 (OLS 3+4)

• /ontologies
• /ontologies/{onto}
• /ontologies/{onto}/classes
• /ontologies/{onto}/classes?iri=https://xxx
• /ontologies/{onto}/classes/{class}
• /ontologies/{onto}/individuals
• /ontologies/{onto}/individuals?iri=https://xxx
• /ontologies/{onto}/individuals/{individual}
• /ontologies/{onto}/entities
• /ontologies/{onto}/entities?iri=https://xxx
• /ontologies/{onto}/classes/{class}/children
• /ontologies/{onto}/properties
• /ontologies/{onto}/properties?iri=https://xxx
• /ontologies/{onto}/properties/{property}
• /ontologies/{onto}/properties/{property}/children
• /entities?iri=https://xxx
• /classes?iri=https://xxx
• /individuals?iri=https://xxx
• /properties (Depends on #123)
• /classes (Depends on #123)
• /individuals (Depends on #123)
• /properties (Depends on #123)
• /stats
• /ontologies/{onto}/classes/{class}/individuals
• /ontologies/{onto}/entities/{entity}
• /ontologies/{onto}/entities/{entity}/relatedFrom
• /ontologies/{onto}/individuals/{individual}/ancestors
• /ontologies/{onto}/classes/{class}/relatedFrom
• /ontologies/{onto}/classes/{class}/hierarchicalDescendants
• /ontologies/{onto}/classes/{class}/hierarchicalChildren
• /ontologies/{onto}/classes/{class}/hierarchicalAncestors
• /ontologies/{onto}/classes/{class}/descendants
• /ontologies/{onto}/classes/{class}/ancestors
• /ontologies/{onto}/properties/{property}/ancestors
• /defined-fields

[rombaum]

As mentioned before. We have more functionality as before. For better testing capacity we should merge and create smaller issues afterwards. For the missing endpoints should also issues should be created.