Internal flag to capture by value in eager.function.Function.

This is accomplished by disallowing creation of "read" operations inside
FuncGraphs when capture_by_value is true, executing those operations
outside the graph instead and bringing the resulting value into the
right context.

This can misbehave somewhat, but should be enough to migrate the few
users of tf.data which rely on capturing by value.

PiperOrigin-RevId: 233147220