opensearchapi: split SnapshotGetResp into sub structs

[Jakob3xD]

Description

Split the SnapshotGetResp into more sub structs so they can be reused by the user.
This is for example needed when looping over SnapshotGetResp.Snapshots and want to give the struct to a function.

This is actually a more general issue affecting more structs that could be solved by code generation in the future.

Issues Resolved

None

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.99%. Comparing base (06a6dc8) to head (21b8b0f).
Report is 60 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #603       +/-   ##
===========================================
+ Coverage   57.29%   67.99%   +10.70%     
===========================================
  Files         315      376       +61     
  Lines        9823     8862      -961     
===========================================
+ Hits         5628     6026      +398     
+ Misses       2902     1555     -1347     
+ Partials     1293     1281       -12     

Flag	Coverage Δ
integration	60.35% <ø> (+9.51%)	⬆️
unit	15.37% <ø> (+2.53%)	⬆️

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

Files	Coverage Δ
opensearchapi/api_snapshot-get.go	100.00% <ø> (ø)

... and 274 files with indirect coverage changes

[dblock]

Rebase after #604?