Add leads of eventing WGs as area approvers (#2295)

There are now owner aliases for eventing wg leads, sources wg leads,
and channels wg leads. Took a first stab at approvers:

Eventing WG leads approve docs/eventing
Sources WG leads approve docs/eventing/sources
Delivery (Channels) WG leads approve docs/eventing/channels

OWNERS_ALIASES

@@ -34,14 +34,29 @@ aliases:
   - tcnghia
   - vagababov
 
+  eventing-eng-approvers:
+  - grantr
+  - lionelvillard
+  - vaikas
+
   eventing-eng-reviewers:
   - evankanderson
   - grantr
-  - vaikas
-  - n3wscott
-  - matzew
-  - nachocano
   - lionelvillard
+  - matzew
+  - n3wscott
+  - nachocano
+  - vaikas
+
+  eventing-channels-eng-approvers:
+  - Harwayne
+  - matzew
+
+  eventing-sources-eng-approvers:
+  - lionelvillard
+  - n3wscott
+  - nachocano
+  - vaikas
 
   install-eng-reviewers:
   - dprotaso

docs/eventing/OWNERS

@@ -1,4 +1,6 @@
 # The OWNERS file is used by prow to automatically merge approved PRs.
 
+approvers:
+- eventing-eng-approvers
 reviewers:
 - eventing-eng-reviewers

docs/eventing/channels/OWNERS

@@ -0,0 +1,4 @@
+# The OWNERS file is used by prow to automatically merge approved PRs.
+
+approvers:
+- eventing-channels-eng-approvers

docs/eventing/sources/OWNERS

@@ -0,0 +1,4 @@
+# The OWNERS file is used by prow to automatically merge approved PRs.
+
+approvers:
+- eventing-sources-eng-approvers