Microbatch: store model context var as dict, not ModelNode

[MichelleArk]

Resolves: #10927
Resolves: dbt-labs/dbt-bigquery#1376

Problem

We were erroneously updating the model jinja context variable to be of type ModelNode after re-compiling the microbatch model, when it should have been a dict. This was causing downstream issues in jinja when attempted to be accessed as a dict (e.g. not iterable, no get method).

Solution

• model.to_dict()
• Refactor microbatch jinja context building into MicrobatchBuilder to centralize where context is being updated + ease of testing.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[github-actions]

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.14%. Comparing base (d07bfda) to head (d685226).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10917      +/-   ##
==========================================
- Coverage   89.17%   89.14%   -0.03%     
==========================================
  Files         183      183              
  Lines       23464    23473       +9     
==========================================
+ Hits        20924    20926       +2     
- Misses       2540     2547       +7     

Flag	Coverage Δ
integration	86.43% <100.00%> (-0.03%)	⬇️
unit	62.08% <62.50%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.08% <62.50%> (+0.02%)	⬆️
Integration Tests	86.43% <100.00%> (-0.03%)	⬇️

[QMalcolm]

🚀 LGTM