services/object: Inline signing service

[cthulhu-rider]

Continues 1e40321. Now sign service is gone.

[codecov]

Codecov Report

Attention: Patch coverage is 7.93651% with 116 lines in your changes missing coverage. Please review.

Project coverage is 22.29%. Comparing base (b9fbe92) to head (c56a885).

Files with missing lines	Patch %	Lines
pkg/services/object/server.go	4.20%	114 Missing ⚠️
cmd/neofs-node/object.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3084      +/-   ##
==========================================
+ Coverage   22.24%   22.29%   +0.05%     
==========================================
  Files         771      770       -1     
  Lines       58950    58806     -144     
==========================================
- Hits        13113    13111       -2     
+ Misses      44951    44809     -142     
  Partials      886      886              

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