Add URI read and write methods to Utf8JsonReader/Writer and JsonElement

[layomia]

There is a strong need for URI support:

• https://github.com/dotnet/corefx/issues/38536
• https://github.com/dotnet/corefx/issues/38568

We should add the following APIs to the reader, writer, and JsonElement:

public ref partial struct Utf8JsonReader
{
    public System.Uri GetUri() { throw null; }
    public bool TryGetUri(out System.Uri value) { throw null; }
}

public sealed partial class Utf8JsonWriter : System.IAsyncDisposable, System.IDisposable
{
    public void WriteString(System.ReadOnlySpan<byte> utf8PropertyName, System.Uri value) { }
    public void WriteString(System.ReadOnlySpan<char> propertyName, System.Uri value) { }
    public void WriteString(string propertyName, System.Uri value) { }
    public void WriteString(System.Text.Json.JsonEncodedText propertyName, System.Uri value) { }
    public void WriteStringValue(System.Uri value) { }
}

public readonly partial struct JsonElement
{
    public System.Uri GetUri() { throw null; }
    public bool TryGetUri(out System.Uri value) { throw null; }
}

cc @ahsonkhan, @steveharter, @JeremyKuhne, @bartonjs

[terrajobst]

We reviewed this today:

• Purpose: people have Uri in their types that want to serialize them, which we already do.
• We don't feel like we need to expose this in the reader/writer. It's also not in JSON.NET.
• For now, close. This can be reopened if there is enough customer evidence that this needed.