api: add HTTP-like object getter

[tatiana-nspcc]

Add GET /get/{cid}/{oid} and HEAD /get/{cid}/{oid} APIs.

Close #109.

[codecov]

Codecov Report

Attention: 242 lines in your changes are missing coverage. Please review.

Comparison is base (d0c6513) 15.50% compared to head (14b20e6) 13.70%.
Report is 3 commits behind head on master.

Files	Patch %	Lines
handlers/objects.go	0.00%	231 Missing ⚠️
handlers/api.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   15.50%   13.70%   -1.81%     
==========================================
  Files          10       10              
  Lines        1838     2080     +242     
==========================================
  Hits          285      285              
- Misses       1523     1765     +242     
  Partials       30       30              

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[roman-khimov]

What are the problems we currently have? Bearer token?

Have you tried neofs-testcases with it?

Looks rather legit otherwise.

[tatiana-nspcc]

@cthulhu-rider, I cannot comment directly here: #114 (comment). But contentType is determined for GET and HEAD in http-gw in different ways, so I've kept it.

[cthulhu-rider]

@tatiana-nspcc

I cannot comment directly here

but why? try again to follow ur #114 (comment), it must lead to the active thread where u should be able to comment. Afaik GH duplicates review comments left in threads in form of inline uncommentable blocks, im tryin to avoid them but sometimes they intrude ;( think u meant them

But contentType is determined for GET and HEAD in rest-gw in different ways

yeah that's for sure, but regardless of this we need to check if payloadSize > 0 in HEAD handler cuz ObjectRangeInit requires length > 0. For empty payload we can fallback to http.DetectContentType(nil) as in GET to keep them in sync