fix(eslint-plugin-template): [prefer-ngsrc] Do not prefer ngsrc for base64-encoded strings

[estevezluis]

Closes #1543

Rule prefer-ngsrc should not error if image src is a base64-encoded string because NgOptimizedImage does not support Base64-encoded strings.

Expected Behavior:
<img src="data:image/png;base64" /> ✅ should not throw an error

<img [src]="'data:image/png;base64'" /> ❌ should throw an error because [src] is a BoundAttribute

<img src="data:image/png;base64" [ngSrc]="'data:image/png;base64'" /> ❌ should throw an error because there is two sources.

[nx-cloud]

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✅ Successfully ran 4 targets

• nx run-many -t e2e-suite --parallel 1
• nx run-many -t test
• nx run-many -t build
• nx run-many -t check-rule-docs

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[abaran30]

@estevezluis I'm unsure why codecov isn't running (maybe CI flake). Would you be able to make one more commit to refresh the CI run? Maybe add a comment to the isSrcBase64Image function explaining that the logic is based on Angular's assertion (with a link?) for future reference.

[codecov]

Codecov Report

Merging #1628 (5d55aca) into main (b489924) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1628      +/-   ##
==========================================
+ Coverage   89.38%   89.39%   +0.01%     
==========================================
  Files         169      169              
  Lines        3174     3178       +4     
  Branches      540      541       +1     
==========================================
+ Hits         2837     2841       +4     
  Misses        199      199              
  Partials      138      138              

Flag	Coverage Δ
unittest	89.39% <100.00%> (+0.01%)	⬆️

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

Files	Coverage Δ
...s/eslint-plugin-template/src/rules/prefer-ngsrc.ts	100.00% <100.00%> (ø)
...-plugin-template/tests/rules/prefer-ngsrc/cases.ts	100.00% <ø> (ø)