Add LookupEntityAttachment & GetEntityAttachment natives (#1653)

Using the virtual `CBaseAnimating::GetAttachment(int, matrix3x4_t &)` was a deliberate choice because virtual offsets are generally easier to maintain than signatures. The `matrix3x4_t` is converted to world position and world angles internally.
Some of the other overloads are also inlined on a few games, making this the best choice.

Since this call can only be used on classes inheriting `CBaseAnimating`, we check if the `DT_BaseAnimating` SendTable exists on the entity, throwing a native error if it doesn't.
This safeguard could be greatly improved with a call to `CBaseEntity::GetBaseAnimating`, but would require more gamedata (maybe something to consider for the future?)

plugins/include/sdktools_functions.inc

@@ -377,3 +377,25 @@ native void EntityCollisionRulesChanged(int entity);
  * @error                   Invalid entity or lack of mod support.
  */
 native void SetEntityOwner(int entity, int owner=INVALID_ENT_REFERENCE);
+
+/**
+ * Returns the index number of a given named attachment.
+ * 
+ * @param entity        The entity index.
+ * @param name          The attachment name.
+ * @return              An attachment index, or 0 if the attachment name is invalid or unused.
+ * @error               Invalid entity or lack of mod support.
+ */
+native int LookupEntityAttachment(int entity, const char[] name);
+
+/**
+ * Returns the world location and world angles of an attachment.
+ * 
+ * @param entity        The entity index.
+ * @param attachment    The attachment index.
+ * @param origin        Destination vector to store the attachment's origin vector.
+ * @param angles        Destination vector to store the attachment's position angle.
+ * @return              True on success, otherwise false.
+ * @error               Invalid entity or lack of mod support.
+ */
+native bool GetEntityAttachment(int entity, int attachment, float origin[3], float angles[3]);