PMM-12913 Migrate api endpoints to REST

[ademidoff]

PMM-12913

Link to the Feature Build: SUBMODULES-3581
Link to the FB integrated with frontend: SUBMODULES-3624

With this PR we migrate our API endpoints to make them more RESTful.

You can track the spec file here.

NOTE: currently the Main/Checks workflow is failing because of this issue, which is unrelated to this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 254 lines in your changes missing coverage. Please review.

Project coverage is 44.49%. Comparing base (108972f) to head (6913e01).

Files	Patch %	Lines
admin/commands/management/add_mysql.go	0.00%	38 Missing ⚠️
admin/commands/management/add_postgresql.go	0.00%	38 Missing ⚠️
admin/commands/management/add_mongodb.go	0.00%	36 Missing ⚠️
...min/commands/management/add_external_serverless.go	0.00%	31 Missing ⚠️
admin/commands/management/add_proxysql.go	0.00%	26 Missing ⚠️
admin/commands/management/add_external.go	0.00%	21 Missing ⚠️
admin/commands/management/add_haproxy.go	0.00%	19 Missing ⚠️
admin/commands/management/remove.go	0.00%	19 Missing ⚠️
admin/commands/inventory/list_agents.go	0.00%	5 Missing ⚠️
admin/commands/inventory/list_services.go	0.00%	4 Missing ⚠️
... and 8 more

Additional details and impacted files

@@                     Coverage Diff                     @@
##           v3-api-breaking-changes    #2880      +/-   ##
===========================================================
- Coverage                    44.68%   44.49%   -0.19%     
===========================================================
  Files                          362      360       -2     
  Lines                        35377    35500     +123     
===========================================================
- Hits                         15808    15797      -11     
- Misses                       17957    18097     +140     
+ Partials                      1612     1606       -6     

Flag	Coverage Δ
admin	11.46% <0.00%> (-0.05%)	⬇️
agent	52.60% <ø> (-0.14%)	⬇️
managed	46.69% <ø> (-0.25%)	⬇️
vmproxy	66.66% <ø> (ø)

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