Improve examples cache

[t0yv0]

Fleshing out the examples cache a bit. With the introduction of PULUMI_CONVERT=1 the build times are making large providers like AWS very unpleasant to work with. In particular examples are being re-rendered repeatedly across make tfgen and make build_sdks as well as across multiple passes through the provider. This PR improves pre-existing examples cache a bit with the following goals in mind:

• a lot more information that might be relevant to examples generation is captured into the cache key
• so that changes in tooling versions that might affect example rendering should invalidate the cache
• cache directories now have a metadata file explaining the details of the cache key
• PULUMI_CONVERT=1 style conversions now support accelerating through the cache
• this is slightly tricky as PULUMI_CONVERT=1 bulk-detects HCLs but now needs to be aware of the cache to skip them
• cache hits should not affect example coverage tracker metrics

[iwahbe]

Looks good to me.

I'd be comfortable disabling gosec. I'm struggling to think of a case where it flagged an error for us.

[iwahbe]

Are we not returning here because we want g.coverageTracker.languageConversionSuccess(e, languageName, convertedHcl) to trigger later down?

[t0yv0]

That's right. Looks like this results in a redundant cache.Store call. I can fix that up.

[codecov]

Codecov Report

Attention: Patch coverage is 88.48921% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 59.21%. Comparing base (e336d98) to head (7a061ae).
Report is 2 commits behind head on master.

Files	Patch %	Lines
pkg/tfgen/convert_cli.go	57.14%	5 Missing and 1 partial ⚠️
pkg/tfgen/examples_cache.go	94.91%	5 Missing and 1 partial ⚠️
pkg/tfgen/docs.go	42.85%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1717      +/-   ##
==========================================
- Coverage   59.62%   59.21%   -0.41%     
==========================================
  Files         299      307       +8     
  Lines       41918    42403     +485     
==========================================
+ Hits        24993    25109     +116     
- Misses      15492    15860     +368     
- Partials     1433     1434       +1     

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