feat(core): Allow partial context in makeOperation (QOL)

[kitten]

Summary

This is a small quality of life improvement for the makeOperation helper in @urql/core that'll mainly benefit users rather than the core code.

When makeOperation is called with an Operation rather than a request, we should spread OperationContext and allow a partial one.

// before
makeOperation(operation.kind, operation, {
  ...operation.context,
  requestPolicy: 'cache-and-network',
});

// after
makeOperation(operation.kind, operation, {
  requestPolicy: 'cache-and-network',
});

## Set of changes

- Allow partial `OperationContext` for when `makeOperation` is called on an `Operation` rather than a `GraphQLRequest`