CSharp bindings: user-defined string encoding and improved safety

[Mbucari]

What does this PR do?

Creates an interface for users to create custom string Encoder/Decoders for Gdal to use for all string operations. This serves a purpose similar to #3825 by allowing users to manage how strings are encoded/decoded to/from Gdal.

Expose $module.IStringEncoder interfaces which have 2 methods:

• string FromNullTerminated(IntPtr pStr); - Read native null-terminated data from a Gdal pointer and return a managed string.
• byte[] ToNullTerminated(string str); - Create a null-terminated array of bytes from a managed string to send to Gdal.

It also simplifies string marshalling changes made in recent PR by no longer requires C# to allocate/free any unmanaged memory. All string/byte[] objects are managed.

---

Here's an example of a custom StringEncoder.

class CustomGdalEncoder : OSGeo.GDAL.Gdal.IStringEncoder, OSGeo.OGR.Ogr.IStringEncoder, OSGeo.OSR.Osr.IStringEncoder
{
    public string FromNullTerminated(IntPtr pStr)
      => Marshal.PtrToStringUTF8(pStr);

    public byte[] ToNullTerminated(string str)
    {
        if (str == null)
            return null;
        int byteCount = Encoding.UTF8.GetByteCount(str);
        var bts = new byte[byteCount + 1];
        Encoding.UTF8.GetBytes(str, 0, str.Length, bts, 0);
        return bts;
    }
}
var encoder = new CustomGdalEncoder();
OSGeo.GDAL.Gdal.StringEncoder = encoder;
OSGeo.OGR.Ogr.StringEncoder = encoder;
OSGeo.OSR.Osr.StringEncoder = encoder;

@runette

What are related issues/pull requests?

#3825
#14283

Tasklist

• Make sure code is correctly formatted
• Review
• Adjust for comments

Environment

Provide environment details, if relevant:
Mono and dotnet

[runette]

It is probably going to be next week before I can look at this