add GCCover contract docs

Author: Max Charlamb

docs/design/datacontracts/GCCover.md

@@ -0,0 +1,29 @@
+# Contract GCCover
+
+This contract encapsulates support for [GCCover](../coreclr/jit/investigate-stress.md) (GC stress testing) in the runtime.
+
+## APIs of contract
+
+```csharp
+public virtual TargetPointer? GetGCCoverageInfo(NativeCodeVersionHandle codeVersionHandle);
+```
+
+## Version 1
+
+Data descriptors used:
+| Data Descriptor Name | Field | Meaning |
+| --- | --- | --- |
+| NativeCodeVersionNode | GCCoverageInfo | pointer to GC coverage info |
+
+Contracts used:
+| Contract Name |
+| --- |
+| CodeVersions |
+| RuntimeTypeSystem |
+
+### Getting GCCoverageInfo for a NativeCodeVersion
+```csharp
+public virtual TargetPointer? GetGCCoverageInfo(NativeCodeVersionHandle codeVersionHandle);
+```
+1. If `codeVersionHandle` is synthetic, attempt to read the GCCoverageInfo off of the MethodDesc using the RuntimeTypeSystem contract.
+2. If `codeVersionHandle` is explicit, fetch the `NativeCodeVersionNode` and attempt to read the `GCCoverageInfo` pointer. This value will only exist on targets with `HAVE_GCCOVER` enabled. If this value exists return it. Otherwise return null.

docs/design/datacontracts/RuntimeTypeSystem.md

@@ -159,6 +159,9 @@ partial interface IRuntimeTypeSystem : IContract
     // Get an instruction pointer that can be called to cause the MethodDesc to be executed
     public virtual TargetCodePointer GetNativeCode(MethodDescHandle methodDesc);
 
+    // Gets the GCCoverageInfo pointer if available.
+    // If not enabled in the target process, returns null.
+    public virtual TargetPointer? GetGCCoverageInfo(MethodDescHandle methodDesc);
 }
 ```