System.Formats.Cbor: Add Reader and Writer methods for System.Half.

[eiriktsarpalis]

Background and Motivation

The CBOR spec requires support for IEEE 754 half-precision floats. Given that we just added System.Half, we should expose reader and writer methods that handle that particular type.

Proposed API

public class CborReader
{
    public void WriteHalf(Half value);
}

public class CborWriter
{
    public Half ReadHalf();
}

Relevant implementation PR: #38946

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