client: merge ExecInspectResult with ExecInspect

The `ExecInspectResult` type was embedding `ExecInspect`, which is also
defined by the client, so there's no need to abstract it.

While updating, also;

- Rename `ExecID` to `ID`, to match the field-name returned by the API.
- Rename `Pid` to `PID`, to be in the right casing.
- Remove `json` labels, as option-types are not (un)marshaled to JSON.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/container_exec_test.go

@@ -141,6 +141,6 @@ func TestExecInspect(t *testing.T) {
 
 	inspect, err := client.ExecInspect(context.Background(), "exec_id", ExecInspectOptions{})
 	assert.NilError(t, err)
-	assert.Check(t, is.Equal(inspect.ExecID, "exec_id"))
+	assert.Check(t, is.Equal(inspect.ID, "exec_id"))
 	assert.Check(t, is.Equal(inspect.ContainerID, "container_id"))
 }