Add validator spec via code generation

[schlessera]

This PR adds an object hierarchy that represents the validator spec.

It downloads the JSON version of the validator spec data and uses code generation to build the object hierarchy as a result. The code generation can be manually retriggered for a refresh via the following command:

php bin/generate-validator-spec.php

The generated validator spec is found under AmpProject\Validator\Spec (in the src/Validator/Spec subfolder), whereas the code generation is found under (the --dev only) AmpProject\Tooling\Validator\SpecGenerator (in the bin/Validator/SpecGenerator subfolder).

[codecov]

Codecov Report

Merging #100 (db0f8be) into main (7d6402e) will decrease coverage by 8.84%.
The diff coverage is 17.78%.

@@             Coverage Diff              @@
##               main     #100      +/-   ##
============================================
- Coverage     80.87%   72.03%   -8.85%     
- Complexity      928     1017      +89     
============================================
  Files            48       65      +17     
  Lines          2311     2739     +428     
============================================
+ Hits           1869     1973     +104     
- Misses          442      766     +324     

Flag	Coverage Δ	Complexity Δ
php	72.03% <17.78%> (-8.85%)	0.00 <74.00> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
src/Amp.php	93.05% <ø> (ø)	43.00 <0.00> (ø)
src/Exception/InvalidAttributeName.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Exception/InvalidCssName.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Exception/InvalidExtension.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Exception/InvalidFormat.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Exception/InvalidListName.php	0.00% <0.00%> (ø)	2.00 <2.00> (?)
src/Exception/InvalidSpecName.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Exception/InvalidSpecRuleName.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Exception/InvalidTagId.php	0.00% <0.00%> (ø)	1.00 <1.00> (?)
src/Validator/Spec/AttributeList.php	0.00% <0.00%> (ø)	4.00 <4.00> (?)
... and 26 more

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[westonruter]

LGTM ✅

Just kidding. As you noted, I'll have to digest this. Maybe we could step through it next week so I can get a high-level overview before getting into the details?

[westonruter]

To do full post-merge review as discussed.

[westonruter]

To do full post-merge review as discussed.

See #221.