Improve pf/internal/convert module

[t0yv0]

Improve pf/internal/convert module

• add NewObjectEncoder and NewObjectDecoder to make it easier to use without a full-blown Encoding
• add InferObjectType to help with testing by computing plausible tftypes.Object instances
• unexport LocalPropertyNames, TerraformPropertyName etc
• add test coverage
• rename propertyValue.go to property_value.go for consistency
• remove unused code

On top of t0yv0/simplify-diff

[codecov]

Codecov Report

Attention: Patch coverage is 77.50000% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 60.03%. Comparing base (c5f6262) to head (04d9db2).
Report is 1 commits behind head on master.

Files	Patch %	Lines
pf/internal/convert/convert.go	58.62%	8 Missing and 4 partials ⚠️
pf/internal/convert/infer_object.go	80.64%	12 Missing ⚠️
pf/tfbridge/provider_diff.go	56.52%	10 Missing ⚠️
pf/internal/convert/encoding.go	94.87%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1779      +/-   ##
==========================================
+ Coverage   59.98%   60.03%   +0.05%     
==========================================
  Files         302      309       +7     
  Lines       42180    42591     +411     
==========================================
+ Hits        25300    25571     +271     
- Misses      15452    15549      +97     
- Partials     1428     1471      +43     

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