Add read command

[lpoli]

A brief description of the changes in this PR: This PR adds read command that uses io.ReadSeekCloser interface to download file from an allocation.

Tasks to complete before merging PR:

• [ ] Ensure system tests are passing. If not Run them manually to check for any regressions :clipboard:
• [ ] Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• [ ] Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• 0chain:
• blobber:
• gosdk: https://github.com/0chain/gosdk/pull/753
• system_test:
• zwalletcli:
• Other: ...

[Kishan-Dhakan]

@lpoli what's the status of this PR?