HDDS-3638. Add a cat command to show the text of a file in the Ozone server

[maobaolong]

What changes were proposed in this pull request?

When we deploy a hdfs cluster, our devops usually want to put a file into the cluster and cat the result to ensure anything is ok, so they want this command of ozone too.

Some time, we only want to print the content of the file in Ozone, we don't need to download it now.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-3638

How was this patch tested?

bin/ozone sh key cat /myvol/mybucket/NOTIC.txt

[adoroszlai]

Thanks @maobaolong for adding this command. It works fine. I have suggestion for a minor improvement in the description. In the future I think we might want to support catting multiple keys (similar to HDFS and Unix cat), but this is a good start.

[xiaoyuyao]

LGTM overall, just some minor comments inline.

[maobaolong]

@adoroszlai @xiaoyuyao Thanks for your review and help, i've addressed your comments, PTAL.

[adoroszlai]

Thanks @maobaolong for updating the patch. I only have a question regarding how @xiaoyuyao's comment was addressed.

[maobaolong]

@adoroszlai Thank you for the clarification, i use a const 4096 as chunk size, i think i have address @xiaoyuyao 's comments now, sorry for the misunderstand.

[adoroszlai]

LGTM. @xiaoyuyao please confirm the buffer size, and then I think this can be merged.

[xiaoyuyao]

LGTM, +1 pending CI.