Backport of add support for attributes with nested types in providers schema into v0.15

[teamterraform]

Backport

This PR is auto-generated from #28055 to be assessed for backporting due to the inclusion of the label 0.15-backport.

The below text is copied from the body of the original PR.

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This PR extends jsonprovider to support attributes with NestedTypes and extends test coverage in jsonprovider and the providers schemas tests. I've also cleaned up some comments and extracted the logic to parse the nesting mode so it can be used in both marshaling blocks and attributes.

Fixes #28050

[codecov]

Codecov Report

Merging #28068 (54bcbeb) into v0.15 (413f020) will increase coverage by 0.01%.
The diff coverage is 66.66%.

Impacted Files	Coverage Δ
command/jsonprovider/block.go	66.66% <41.17%> (+0.95%)	⬆️
command/jsonprovider/attribute.go	92.00% <100.00%> (+5.33%)	⬆️
backend/remote/backend_common.go	50.35% <0.00%> (-0.72%)	⬇️
dag/marshal.go	63.49% <0.00%> (+1.58%)	⬆️
internal/providercache/dir.go	73.46% <0.00%> (+6.12%)	⬆️

[ghost]

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