Fix NETStandard library using JSON source gen

[ericstj]

Fixes #61737

NETStandard libraries using JSON source gen would fail to load on
.NETCore due to missing IsExternalInit in the assembly.

On .NETCore this is defined, whereas on .NETStandard JSON carries an
internal copy. The compiler emits references to the internal type when
a NETStandard library calls init-only setters in JSON types, but these
references are not satisfied when the library runs on .NETCore.

Fix this by adding a type forward to JSON for the IsExternalInit type.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

---

Fixes #61737

NETStandard libraries using JSON source gen would fail to load on
.NETCore due to missing IsExternalInit in the assembly.

On .NETCore this is defined, whereas on .NETStandard JSON carries an
internal copy. The compiler emits references to the internal type when
a NETStandard library calls init-only setters in JSON types, but these
references are not satisfied when the library runs on .NETCore.

Fix this by adding a type forward to JSON for the IsExternalInit type.

Author:	ericstj
Assignees:	-
Labels:	area-System.Text.Json, new-api-needs-documentation
Milestone:	-

[ericstj]

I could have chosen just 1 copy of the generator to use, or even just checked in the generated code, but I created a parallel structure in case you find it interesting to add more tests. Let me know if you think this is overkill.

[eerhardt]

This LGTM. I had some nits and questions.

[ericstj]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1669729149

[github-actions]

@ericstj backporting to release/6.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Fix NETStandard library using JSON source gen
Using index info to reconstruct a base tree...
M	src/libraries/System.Text.Json/ref/System.Text.Json.csproj
M	src/libraries/System.Text.Json/src/System.Text.Json.csproj
Falling back to patching base and 3-way merge...
Auto-merging src/libraries/System.Text.Json/src/System.Text.Json.csproj
CONFLICT (content): Merge conflict in src/libraries/System.Text.Json/src/System.Text.Json.csproj
Auto-merging src/libraries/System.Text.Json/ref/System.Text.Json.csproj
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Fix NETStandard library using JSON source gen
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!