Collect .NET coverage info (#2023)

## Summary of the Pull Request

Collect .NET coverage during build and display it in GitHub Summary.

## Info on Pull Request

Add the `reportgenerator` tool, run it after tests.

## Validation Steps Performed

Run workflow and make sure it works.

See example coverage output here: https://github.com/microsoft/onefuzz/actions/runs/2458088617

src/ApiService/.config/dotnet-tools.json

@@ -0,0 +1,12 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "dotnet-reportgenerator-globaltool": {
+      "version": "5.1.9",
+      "commands": [
+        "reportgenerator"
+      ]
+    }
+  }
+}