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: https://github.com/dbt-labs/dbt-core/pull/10238

Checklist

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

[github-actions[bot]]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/dbt-labs/dbt-common/pull/141/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/dbt-labs/dbt-common/pull/141/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `64.39% <0.00%> (-0.40%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs#carryforward-flags-in-the-pull-request-comment) to find out more.

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