Ensure source generated metadata properties are read-only.

[eiriktsarpalis]

Alternative PR to #76540 which includes the full-blown freezing functionality on the type itself:

namespace System.Text.Json.Metadata;

public partial static class JsonTypeInfo
{
    public bool IsReadOnly { get; }
    public void MakeReadOnly();
}

Fixes #76535

[dotnet-issue-labeler]

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]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Alternative PR to #76540 which includes the full-blown freezing functionality on the type itself:

namespace System.Text.Json.Metadata;

public partial static class JsonTypeInfo
{
    public bool IsReadOnly { get; }
    public void MakeReadOnly();
}

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