C#: Expose byte array compress and decompress

[beicause]

Expose the C# equivalent to PackedByteArray.compress, PackedByteArray.decompress and PackedByteArray.decompress_dynamic.

[paulloz]

Hi! Thank you for your contribution.
This is indeed missing from the C# API, and I think some compression methods might not be available through System.IO.Compression. We'd rather have the new methods defined in GD, though, alongside the implementations of IsEmpty(), Join(), and Stringify().

[paulloz]

Works fine, and looks good to me!

@raulsntos do we want to make compressionMode optional, and default to FileAccess.CompressionMode.Fastlz like the GDScript API?

[beicause]

Just my opinion, I think it's less than ideal to make the argument optional. In addition, fastlz is a relatively old compression and doesn't have much advantages compared to modern compression such as zstd or lz4. https://github.com/inikep/lzbench/blob/master/doc/lzbench20_sorted.md.
But if .NET team thinks it's more important to be consistent with GDScript, I have no objection either.

[raulsntos]

I think it makes sense to match unless we had a strong reason not to, but I don't have a strong opinion either way.

[Repiteo]

Thanks!