Implement XQuery method

[synechist]

Note: pending testing of XQuery+EXR autobuild

go-xrouter shall support a method to query the GraphQL endpoint(s) of XQuery services.

Relevant XQuery specifications:

• Only 1 graph upon launch of XQuery. Expected endpoint is [publicIP]:[port]/xrs/eth_passthrough/pangolinTrades/v1
• Other graphs anticipated to launch very soon
• 1 main endpoint per graph
• additional /help endpoint per graph, providing the graph's schema (i.e. a basic introspection query)

Requirements

• upon launch, go-xrouter shall obtain a list of XQuery graphs supported by service nodes in getNetworkServices and related requests.
• go-xrouter shall employ a single method to interact with any available graphs.