feat: add entry flags

[Sherlock-Holo]

add flags field for Entry

in io-uring, the cqe flags is meaningful, add this field can let driver use the flags

in IOCP or epoll, this flags can be simply set to 0

[Berrysoft]

Would you point out clearer that what kind of operation will set this flag?

[Sherlock-Holo]

for example:

• Recv

when use Recv with buf_group to use provider buffers/register buffers, you should check the cqe flags to get the buffer id, that is the example (the example use Read but Recv is also use cqe flags to get buffer id)

• RecvMulti

the RecvMulti must use provider buffers/register buffers, so you need to check the cqe flags too

[Berrysoft]

We don't support pushing custom squeue::Entry to the driver directly now, so the cases you mentioned is not practical.

This feature should be together with a PR which adds registered buffer support or *Multi opcode support.

[George-Miao]

@Sherlock-Holo Should this flag also be exposed to higher level results? Seems like currently we don’t have way to expose such flags.

[Sherlock-Holo]

adding registered buffer support or *Multi opcode support is more complicated than this, adding flags field is the first step

[Berrysoft]

A simple change without usage is unnecessary.

[George-Miao]

Will there be further update on this PR?

[Berrysoft]

Suppressed by #102