FileInfo functions

Add 2 new functions that work with files (https://github.com/microsoft/Power-Fx/blob/main/src/libraries/Microsoft.PowerFx.Core/Public/Values/BlobValue.cs)

These don't need to be included by default yet. But can be pulled in via:
Config.EnableFileFunctions()

FileInfo

New function, called "FileInfo", which takes a blob and returns a record of file information:

FileInfo(Blob): FileInfo

Where:

FileInfo = Type(
{
    Name: Text,
    Size: Decimal, // 53 bits is large enough 
    MIMEType: Text,
    URL: Text
});

Blob is a File type in Power Fx (Dtype.Blob)
Note: Power Fx only has Floating point /Decimal numberic types (no integer types).

Text(Blob) // should return appres URL, not impl in C# interpreter yet.

// semantically same as:
FileInfo(Blob).URL

PatchFileInfo

PatchFileInfo( File: Blob, UpdatedInfo: FileInfo ): Blob

For changing the file name, it would be PatchFileInfo( MyFile, { Name: "My new file name" } ).

Won't allow updating Size/Url. Runtime error.
So this would be an error:
PatchFileInfo(Blob, FileInfo(blob))

Open Issues:

1. Size is Number of Decimal?
2. Final name of PatchFileInfo

Implementation plan

1. In C# side, we define new virtuals on BlobValue for GetFileInfo, SetFileInfo.
2. Power Fx side defines the two functions above
3. Their impl will call the new virtuals on BlobValue

Hosts are already derive from BlobValue, and so you would just implement those virtuals as you see fit.

Start with src/libraries/Microsoft.PowerFx.Core/Public/Values/BlobValue.cs and review the proposed GetFileInfo and SetFileInfo virtuals. Trace how Config.EnableFileFunctions() exposes functions and how BlobValue hosts are implemented. Done means FileInfo and PatchFileInfo are defined with the specified Blob and record behavior, including runtime errors for unsupported Size and URL updates.