Allow snapshots to be defined with YAML only.

[peterallenwebb]

Resolves #10246

Problem

Users had to awkwardly create jinja snapshot blocks in SQL files, even for very simple snapshots.

Solution

Delight users by allowing snapshots to be defined intuitively, with just a few lines of YAML.

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.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.97%. Comparing base (db69473) to head (7bf392e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10762   +/-   ##
=======================================
  Coverage   88.97%   88.97%           
=======================================
  Files         181      181           
  Lines       22956    22977   +21     
=======================================
+ Hits        20424    20444   +20     
- Misses       2532     2533    +1     

Flag	Coverage Δ
integration	86.15% <90.47%> (+<0.01%)	⬆️
unit	62.39% <4.76%> (-0.06%)	⬇️

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

Components	Coverage Δ
Unit Tests	62.39% <4.76%> (-0.06%)	⬇️
Integration Tests	86.15% <90.47%> (+<0.01%)	⬆️

[gshank]

Looks good!