Feature: Standardize the type of fetch policy

ReactRelayQueryRenderer uses this as a type of fetchPolicy:

```
const NETWORK_ONLY = 'NETWORK_ONLY';
const STORE_THEN_NETWORK = 'STORE_THEN_NETWORK';
const DataFromEnum = {
NETWORK_ONLY,
STORE_THEN_NETWORK,
};
type DataFrom = $Keys;
```
while

ReactRelayRefetchContainer uses this as a type of fetchPolicy (in ReactRelayTypes.js):

```
export type RefetchOptions = {|
+force?: boolean,
+fetchPolicy?: 'store-or-network' | 'network-only',
|};
```

It would be more convenient to also have DataFrom in ReactRelayTypes and change RefetchOptions like this:

```
export type RefetchOptions = {|
+force?: boolean,
+fetchPolicy?: DataFrom,
|};
```

Start by comparing the fetchPolicy definitions in ReactRelayQueryRenderer and ReactRelayTypes.js, especially RefetchOptions and the existing DataFrom values. Trace their usages to confirm the accepted policy names, then standardize the shared type in ReactRelayTypes.js and verify that both consumers remain type-correct.

Written by the indexing model from the issue text.