Add nexchange partner plugin

Port the nexchange reporting plugin (queries https://api.n.exchange audit-orders
endpoint with the x-api-key header) from partner PR #217/#212, without the unrelated
changenow/rango/sideshift refactors bundled in those PRs. Wire it into queryEngine and
the demo partner list.

Author: j0ntz

CHANGELOG.md

@@ -6,6 +6,7 @@
 - added: Add Revolut fiat payment provider
 - added: Add Swapter reporting
 - added: Add NYM Swap (nymswap) reporting
+- added: Add nexchange partner plugin
 - changed: Update sideshift plugin with new optional API fields
 - changed: Query both old and new Sideshift affiliate accounts and merge completed orders to preserve full shift history across an affiliate-account rotation
 - changed: Add signature header support to Exolix

src/demo/partners.ts

@@ -97,6 +97,10 @@ export default {
     type: 'fiat',
     color: '#7214F5'
   },
+  nexchange: {
+    type: 'swap',
+    color: '#1D31B6'
+  },
   nymswap: {
     type: 'swap',
     color: '#FB6E4E'