[netstack] Move SO_REUSEPORT and SO_REUSEADDR option to SocketOptions.

This changes also introduces:
- `SocketOptionsHandler` interface which can be implemented by endpoints to
  handle endpoint specific behavior on SetSockOpt. This is analogous to what
  Linux does.
- `DefaultSocketOptionsHandler` which is a default implementation of the above.
  This is embedded in all endpoints so that we don't have to uselessly
  implement empty functions. Endpoints with specific behavior can override the
  embedded method by manually defining its own implementation.

PiperOrigin-RevId: 343158301

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