Feat/playlist apis

[cnlangzi]

Changes

• added playlist apis: load latest playlist and load playlist file

Fixes

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• 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:
• gosdk:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

Merging #814 (54a52d3) into staging (723aa4a) will decrease coverage by 0.05%.
The diff coverage is 24.64%.

@@ Coverage Diff @@ ## staging #814 +/- ## =========================================== - Coverage 24.37% 24.31% -0.06%  =========================================== Files 77 80 +3 Lines 7968 8084 +116 =========================================== + Hits 1942 1966 +24  - Misses 5756 5843 +87  - Partials 270 275 +5 

Flag	Coverage Δ
Unit-Tests	24.31% <24.64%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...de/go/0chain.net/blobbercore/reference/playlist.go	0.00% <0.00%> (ø)
code/go/0chain.net/blobbercore/reference/ref.go	0.00% <0.00%> (ø)
...0chain.net/blobbercore/handler/handler_playlist.go	28.81% <28.81%> (ø)
...de/go/0chain.net/blobbercore/handler/authticket.go	40.00% <40.00%> (ø)
code/go/0chain.net/blobbercore/handler/context.go	34.93% <50.00%> (ø)
code/go/0chain.net/blobbercore/handler/handler.go	56.69% <100.00%> (+0.20%)	⬆️
...0chain.net/blobbercore/handler/handler_hashnode.go	57.14% <100.00%> (ø)
...ain.net/blobbercore/handler/handler_writemarker.go	64.70% <100.00%> (ø)
.../0chain.net/blobbercore/handler/storage_handler.go	23.34% <100.00%> (+0.05%)	⬆️

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[peterlimg]

lgtm