Allow requests with charset encoding type wrapped in quotes

[volllly]

We should also accept requests which send the Content-Type header like that Content-Type: application/json; charset="UTF-8" instead of that Content-Type: application/json; charset=UTF-8.

We should investigate if the newer .NET versions (7 or 8) accept these headers and if yes check if we can use their solutions as a baseline.

Possible solution:

Create our own EncodingProvider that wraps the default EncodingProvider and trims quotes " and ' from the encoding name in the GetEncoding(string) method. We can register this EncodingProvider using Encoding.RegisterProvider(EncodingProvider)

[volllly]

Also check how the signaturecloud for the at/fr and de markets handle this Content-Type Content-Type: application/json; charset="UTF-8"