Use same DTOs for tequilapi server and client (identity)

[tadaskay]

The same model will also be used in SSE.

Ref: #1844

[codecov-io]

Codecov Report

Merging #1906 into master will decrease coverage by 0.03%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #1906      +/-   ##
==========================================
- Coverage   47.48%   47.45%   -0.04%     
==========================================
  Files         287      287              
  Lines       11992    11945      -47     
==========================================
- Hits         5695     5668      -27     
+ Misses       5832     5816      -16     
+ Partials      465      461       -4     

Impacted Files	Coverage Δ
tequilapi/client/client.go	7.35% <0.00%> (+0.28%)	⬆️
tequilapi/client/dto.go	0.00% <ø> (ø)
tequilapi/endpoints/identities.go	36.60% <88.88%> (-11.02%)	⬇️
nat/traversal/pinger.go	75.73% <0.00%> (-1.48%)	⬇️
core/policy/oracle.go	91.89% <0.00%> (+5.40%)	⬆️

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