[Types] PageInfo types are incorrect (should be read-only)

**Issue**
The types for `PageInfo` under [ConnectionInterface](https://github.com/facebook/relay/blob/main/packages/relay-runtime/handlers/connection/ConnectionInterface.js#L25) should be read-only.

**Example**
`ConnectionInterface.js` they are writable
```
export type PageInfo = {
endCursor: ?string,
hasNextPage: boolean,
hasPreviousPage: boolean,
startCursor: ?string,
...
};
```
But if you run the relay compiler and generate types, the `PageInfo` for those types is read-only:
```
+pageInfo: {|
+startCursor: ?string,
+hasNextPage: boolean,
+endCursor: ?string,
|},
```
Any suggestions for how we should fix this? At the moment, we simply set those types ourselves.

Thank you for the help,