Enforce .Elem.Fields (instead of .Fields) for object types

[iwahbe]

This is a reversal of our previous stance that .Fields should be used. This aligns the
fields argument with schema generation.

We need to choose .Elem.Fields as our 1 representation (instead of .Fields) because
of an ambiguity in shim.Schema. From a comment in the PR:

To prevent confusion, users are barred from specifying
information on ps directly, they must set .Fields on ps.Elem:
ps.Elem.Fields.

We need to make this choice (instead of having users set
information on .Fields (and forbidding ps.Elem.Fields) because
the [shim.Schema] representation is not capable of
distinguishing between Map[Object] and Object. SDKv{1,2}
providers are not able to express Map[Object], but Plugin
Framework based providers are.

While this is technically a breaking change, I don't expect users to be broken. Any user
that would break on this change would have been unable to addapt the last several bridge
versions (for example: pulumiverse/pulumi-talos#99).

Fixes #2185

I have tested this change on talos, AWS, GCP and Azure and it doesn't trigger an error.

[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 6 lines in your changes missing coverage. Please review.

Project coverage is 56.93%. Comparing base (992ffd4) to head (5c7dcd1).
Report is 3 commits behind head on master.

Files	Patch %	Lines
pkg/tfgen/generate.go	58.33%	4 Missing and 1 partial ⚠️
pkg/tfbridge/info/validate.go	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2309      +/-   ##
==========================================
+ Coverage   56.86%   56.93%   +0.06%     
==========================================
  Files         365      365              
  Lines       49962    49957       -5     
==========================================
+ Hits        28413    28444      +31     
+ Misses      20004    19972      -32     
+ Partials     1545     1541       -4     

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[VenelinMartinov]

Can we add some tfgen type override integration tests? We need one for 2185 as well as some tests around Set, List, Object, Map[Object], Map etc.

[iwahbe]

Can we add some tfgen type override integration tests? We need one for 2185 as well as some tests around Set, List, Object, Map[Object], Map etc.

I have added integration tests. See Add integration tests.

[VenelinMartinov]

thanks for adding the tests!

[VenelinMartinov]

looks good - two last small comments