Create separate metadata entries for composite values

[lkubb]

What does this PR do?

• Fixes a crash when accessing out-of-bounds list items in templates
• Implements the workaround for composite values in metadata described in the linked issue/discussion

What issues does this PR fix or reference?

Fixes: #106
Ref: #104

Previous Behavior

• No workable path to multiple roles per minion in templated policies
• Hard crash when accessing out-of-bounds list items

New Behavior

• Provides a somewhat workable path to multiple roles per minion in templated policies
• Accessing out-of-bounds list items is treated like accessing undefined keys (prints a warning)

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.82%. Comparing base (f7204c9) to head (ab3fbc3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/saltext/vault/runners/vault.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
- Coverage   90.83%   90.82%   -0.01%     
==========================================
  Files          79       79              
  Lines       13231    13256      +25     
  Branches     1224     1227       +3     
==========================================
+ Hits        12018    12040      +22     
- Misses        953      956       +3     
  Partials      260      260              

Flag	Coverage Δ
Linux	90.75% <93.33%> (-0.01%)	⬇️
macOS	66.39% <70.00%> (-0.01%)	⬇️
project	80.47% <86.66%> (-0.02%)	⬇️
py310	90.76% <93.33%> (-0.03%)	⬇️
py39	85.49% <93.33%> (+0.01%)	⬆️
salt_3006_9	85.56% <93.33%> (+<0.01%)	⬆️
salt_3007_1	90.72% <93.33%> (-0.03%)	⬇️
tests	95.99% <100.00%> (+<0.01%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.