Shrink upb_MiniTableExtension by removing sub member

Remove the `sub` union member from `struct upb_MiniTableExtension` so
that extensions without sub-messages or closed enums do not pay for
unused sub storage. When an extension has a sub, it is stored in trailing
`upb_MiniTableSubInternal` storage directly after the
`upb_MiniTableExtension`, described by the new internal
`upb_MiniTableExtensionWithSub` type.

Details:

*   The MiniDescriptor decoder computes the trailing sub offset
    (`submsg_ofs`) for the target platform's pointer size. The minitable
    generator emits it via `UPB_SIZE(ofs32, ofs64)`, so 32-bit builds get the
    correct offset. The decoder writes the sub only when building for the
    native platform.
*   The minitable generator declares `<ext>_obj` as either
    `upb_MiniTableExtensionWithSub` or a bare `upb_MiniTableExtension`, and
    emits a `UPB_STATIC_ASSERT` that the sub's offset in
    `upb_MiniTableExtensionWithSub` matches `submsg_ofs` on every platform.
*   Add `_upb_MiniTableField_Sub()`, which locates a field's sub from
    `submsg_ofs`, and use it everywhere that calculation was duplicated
    (MiniTable sub getters, extension sub setters, `upb_MiniTable_SetSub*()`,
    and the MiniDescriptor decoder).
*   Add `_upb_MiniTableField_HasSub()` for the "sub-message or closed enum"
    check used to decide whether a field has a sub.
*   `upb_FieldDef` builds extension MiniTables with
    `_upb_MiniTableExtension_Build()` instead of duplicating the
    "has sub" logic and the allocation size.

PiperOrigin-RevId: 999800168