Support for additional AQL/Query endpoints

[tjoubert]

Added support for the following endpoints:
POST: /_api/explain: Explain an AQL query
POST: /_api/query: Parse an AQL query
DELETE: /_api/query/slow: Clears the list of slow AQL queries
GET: /_api/query/slow: Returns the list of slow AQL queries
DELETE: /_api/query/{query-id}: Kills a running AQL query
DELETE: /_api/query-cache: Clears any results in the AQL query cache
GET: /_api/query-cache/entries: Returns the currently cached query results
GET: /_api/query-cache/properties: Returns the global properties for the AQL query cache
PUT: /_api/query-cache/properties: Globally adjusts the AQL query result cache properties
GET: /_api/query/current: Returns the currently running AQL queries
GET: /_api/query/properties: Returns the properties for the AQL query tracking
PUT: /_api/query/properties: Changes the properties for the AQL query tracking

[Zyqsempai]

@rossmills99 @DiscoPYF PTAL

[Zyqsempai]

@tjoubert Could you please update https://github.com/Actify-Inc/arangodb-net-standard/blob/master/project/roadmap.md correspondingly to previous and this PR's

[tjoubert]

@tjoubert Could you please update https://github.com/Actify-Inc/arangodb-net-standard/blob/master/project/roadmap.md correspondingly to previous and this PR's

@Zyqsempai, I've updated the roadmap doc. Thanks.

[DiscoPYF]

Thank you for keeping the contributions going. I should be able to take a look later this week.

[DiscoPYF]

@tjoubert Thank you for another great set of changes 🥳

I have left a bunch of minor comments. Let me know what you think.

AqlFunctionApiClient was originally for the AQL User Functions Management API but the name "AqlFunction" is generic enough that it makes sense to add other AQL queries endpoints. Ideally we could rename the class to AqlQueriesApiClient but that would be a breaking change for people using this class.

Another thing I noted: I think it would be clearer to keep a List<T> as return type for the endpoints instead of inheriting from List<T> with a response class, so that callers can clearly identify that the return type is a collection.