feat: allow to specify a post's tags

[RiccardoM]

Description

This PR allows to specify a post's tag for categorization. This way developers will be able to categorize and search posts (using third party tools) based on the associated tags.

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #942 (786c8ae) into master (8186cab) will decrease coverage by 0.10%.
The diff coverage is 70.89%.

❗ Current head 786c8ae differs from pull request most recent head 6d0db90. Consider uploading reports for the commit 6d0db90 to get more accurate results

@@            Coverage Diff             @@
##           master     #942      +/-   ##
==========================================
- Coverage   82.74%   82.64%   -0.11%     
==========================================
  Files         166      167       +1     
  Lines       14004    14142     +138     
==========================================
+ Hits        11588    11688     +100     
- Misses       1943     1976      +33     
- Partials      473      478       +5     

Impacted Files	Coverage Δ
x/posts/module.go	8.00% <0.00%> (-0.34%)	⬇️
x/posts/legacy/v3/store.go	69.15% <69.15%> (ø)
x/posts/types/models.go	77.37% <75.00%> (+0.52%)	⬆️
x/posts/keeper/msg_server.go	89.76% <100.00%> (+0.03%)	⬆️
x/posts/types/msgs.go	96.96% <100.00%> (+0.13%)	⬆️
x/reactions/keeper/msg_server.go	96.38% <0.00%> (-0.02%)	⬇️

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[dadamu]

Overall looks good to me, just some minor things in test can be improved.

[leobragaz]

Ready to go