MM-11292: clean up plugins GoDoc (#9109)

* clean up plugins GoDoc:

- eliminate plugin.NewBlankContext() as unnecessary
- export ValidIdRegex as a string vs. the less readable var
- add/update various documentation strings
- hide everything by default, except where used by client plugins or the mattermost-server. The exception to this rule are the `*(Args|Returns)` structs which must be public for go-plugin, but are now prefixed with `Z_` with a warning not to use.
- include a top-level example to get plugin authors started

This is not a breaking change for existing plugins compiled against
plugins-v2.

* remove commented out ServeHTTPResponseWriter

* update examples to match developer docs

* add missing plugin/doc.go license header

plugin/supervisor_test.go

@@ -73,7 +73,7 @@ func testSupervisor(t *testing.T) {
 		ConsoleLevel:  "error",
 		EnableFile:    false,
 	})
-	supervisor, err := NewSupervisor(bundle, log, &api)
+	supervisor, err := newSupervisor(bundle, log, &api)
 	require.NoError(t, err)
 	supervisor.Shutdown()
 }
@@ -92,7 +92,7 @@ func testSupervisor_InvalidExecutablePath(t *testing.T) {
 		ConsoleLevel:  "error",
 		EnableFile:    false,
 	})
-	supervisor, err := NewSupervisor(bundle, log, nil)
+	supervisor, err := newSupervisor(bundle, log, nil)
 	assert.Nil(t, supervisor)
 	assert.Error(t, err)
 }
@@ -111,7 +111,7 @@ func testSupervisor_NonExistentExecutablePath(t *testing.T) {
 		ConsoleLevel:  "error",
 		EnableFile:    false,
 	})
-	supervisor, err := NewSupervisor(bundle, log, nil)
+	supervisor, err := newSupervisor(bundle, log, nil)
 	require.Error(t, err)
 	require.Nil(t, supervisor)
 }
@@ -141,7 +141,7 @@ func testSupervisor_StartTimeout(t *testing.T) {
 		ConsoleLevel:  "error",
 		EnableFile:    false,
 	})
-	supervisor, err := NewSupervisor(bundle, log, nil)
+	supervisor, err := newSupervisor(bundle, log, nil)
 	require.Error(t, err)
 	require.Nil(t, supervisor)
 }