JSONEncoder should use Stream, support leaveOpen on close

[mregen]

Type of Issue

• Bug

Describe the Issue

Constructor allows external StreamWriter to use:

public JsonEncoder(
            IServiceMessageContext context,
            bool useReversibleEncoding,
            StreamWriter writer = null,
            bool topLevelIsArray = false)

, but the Streamwriter is disposed in Close.

--> Update: use Stream and leaveopen in the signature, mark this constructor as obsolete
--> for back compat leave dispose in close, but support leaveOpen for external stream.

To Reproduce

use ext.StreamWriter, try to reuse Streamwriter after encoding, need to create a new StreamWriter, but disposed object cannot be reused.

Expected behavior

external Streamwriter is not disposed, only internal.

Code snippets

            m_writer.Dispose();  // only dispose if streamwriter is owned by JSONEncoder.
            m_writer = null;
            return length;

Environment (please complete the following information):

• OS: any
• Development environment: Visual Studio 2019
• Runtime: .NET Core 3.1
• Component: JSONEncoder, Core

[mregen]

some more issues found while working on #1448 👍🏼

-constructor signature of encoders should allow for similar use..
-use Stream instead of Streamwriter in constructor