add user agent header to request to graphql api

[coret]

To distinguish between requestors to the GraphQL API of the Network of Terms (for statistics and pro-active support) the use the HTTP User-Agent request header seems a good option. To "identify" the requests by the demonstrator, each request should include a User-Agent, like for example network-of-terms-demo/1.0 (+https://termennetwerk.netwerkdigitaalerfgoed.nl/)