Add `--file` flag for `docker secret create` command
Add `--file` flag for `docker secret create` command

This fix tries to address the issue raised in 28581 and 28927
where it is not possible to create a secret from a file (only
through STDIN).

This fix add a flag `--file` to `docker secret create` so that
it is possible to create a secret from a file with:
```
docker secret create --file secret.in secret.name
```

or
```
echo TEST | docker secret create --file - secret.name
```

Related docs has been updated.

An integration test has been added to cover the changes.

This fix fixes 28581.
This fix is related to 28927.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

docker/cli,ashfall/cli,docker/cli,riyazdf/cli,Zebrilee/cli,e11137/cli,cpuguy83/cli,riyazdf/cli,allencloud/cli,allencloud/cli,dnephin/cli,redpanda/cli,redpanda/cli,Zebrilee/cli,e11137/cli,cpuguy83/cli,thaJeztah/cli,dnephin/cli,ashfall/cli,thaJeztah/cli,gdevillele/cli