Move StatsItem, StatsDict, TableMetadata to dbt-common

[aranke]

Description

As per internal Slack conversation, we want to move the catalog artifact schema to dbt-common so adapters can depend on them (PR 1, PR 2).

This PR moves the following to dbt-common: StatsItem, StatsDict, TableMetadata
Draft PR in dbt-core that keeps imports intact but uses code from dbt-common is here: dbt-labs/dbt-core#10238

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 64.39%. Comparing base (315aeae) to head (5caf8cb).

Files	Patch %	Lines
dbt_common/contracts/metadata.py	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
- Coverage   64.78%   64.39%   -0.40%     
==========================================
  Files          50       51       +1     
  Lines        3115     3134      +19     
==========================================
  Hits         2018     2018              
- Misses       1097     1116      +19     

Flag	Coverage Δ
unit	64.39% <0.00%> (-0.40%)	⬇️

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[MichelleArk]

One comment regarding the location of these classes but otherwise looks good! Thank you :)