PMM-11456: Implement backend part for onboarding tips

[dliakhov]

PMM-11456:

If this PR adds or removes or alters one or more API endpoints, please review and add or update the relevant API documents as well:

• [ ] API Docs updated

If this PR is related to some other PRs in this or other repositories, please provide links to those PRs:

• Links to related pull requests (optional).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 67.15543% with 112 lines in your changes missing coverage. Please review.

Project coverage is 43.01%. Comparing base (523ab2f) to head (0758bf4). Report is 607 commits behind head on main.

Files	Patch %	Lines
managed/services/onboarding/tips.go	63.23%	52 Missing and 23 partials :warning:
managed/models/onboarding_system_tip_reform.go	75.00%	13 Missing :warning:
managed/models/onboarding_user_tip_reform.go	78.33%	13 Missing :warning:
managed/models/onboarding_system_tip.go	40.00%	6 Missing :warning:
managed/cmd/pmm-managed/main.go	0.00%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2216      +/-   ##
==========================================
+ Coverage   42.89%   43.01%   +0.11%     
==========================================
  Files         384      389       +5     
  Lines       47517    47858     +341     
==========================================
+ Hits        20382    20584     +202     
- Misses      25247    25362     +115     
- Partials     1888     1912      +24     

Flag	Coverage Δ
admin	10.51% <ø> (+0.04%)	:arrow_up:
agent	52.41% <ø> (-0.30%)	:arrow_down:
managed	44.41% <67.15%> (+0.23%)	:arrow_up:
vmproxy	70.58% <ø> (ø)

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[ademidoff]

@dliakhov Just one test left to fix - TestTipsServiceCompleteUserTip

[dliakhov]

@dliakhov Just one test left to fix - TestTipsServiceCompleteUserTip

Done