Add optional context argument to ArtemisClient

[matehat]

What does this PR do/solve?

This non-breaking change to ArtemisClient allows a caller to provide additional Context information to queries (on both execute or stream).

For example, the following would add a request-specific header to a GraphQL query, just before sending it:

import 'package:graphql/client.dart' as graphql;

final requestUuid = Uuid().v4();
artemisClient.execute(
  query, 
  context: graphql.Context.fromList([
    graphql.HttpLinkHeaders(headers: {'x-request-id': requestUuid})
  ]),
);

/cc @Pacane

[matehat]

Any chance we can consider this PR?

@comigor

[comigor]

oh wow, we've merged basically the same code a couple of months ago #324 (available since 7.0.0-beta.14)

[matehat]

Actually glad to hear it! Thanks