refactor: abstract common api client

[Ace-Tang]

Signed-off-by: Ace-Tang aceapril@126.com

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how you did it

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[allencloud]

CI fails

# github.com/alibaba/pouch/test/request
request/request.go:118:2: cannot use client.CommonAPIClient as type *client.APIClient in return argument: need type assertion
make: *** [test] Error 2

And could you tell us more about the reason why you did this? @Ace-Tang

[Ace-Tang]

I did this pr for two reason.

1. In package client , someone has abstracted container and image api client as ContainerAPIClient and ImageAPIClient
2. I thought that abstract these api client as a common interface is a good idead, so I did this.

WDYT, @allencloud , the pr is not necessary.

[allencloud]

To be honest, I would definitely say that this PR is very important for the project. Although it brings less effect compared to the original one, consistency and reader-friendly is very important for an open source project. I must 👍 for your idea. @Ace-Tang

[codecov-io]

Codecov Report

Merging #740 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #740   +/-   ##
=======================================
  Coverage   13.43%   13.43%           
=======================================
  Files          99       99           
  Lines        5992     5992           
=======================================
  Hits          805      805           
  Misses       5133     5133           
  Partials       54       54

Impacted Files	Coverage Δ
client/container.go	0% <ø> (ø)	⬆️
cli/cli.go	0% <ø> (ø)	⬆️
client/image.go	0% <ø> (ø)	⬆️
client/client.go	80% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c31211f...9f3c119. Read the comment docs.

[allencloud]

LGTM, thanks a lot for the work. @Ace-Tang