feat: Add data:application/json;base64 support to AuthenticationOauth2

[chriscameron-vertexinc]

Added support for using OAuth2 with a base64 encoded string instead of a file path.

Workaround for #253

[Lanayx]

I would write it as

match privateKey.Scheme with
| "file" ->
    openAndDeserializeCreds privateKey.LocalPath
| "data" ->
    decodeAndDeserializeCreds privateKey |> Task.FromResult
| _ ->
    raise <| NotSupportedException $"Scheme '{privateKey.Scheme}' is not supported."

and would also skip .ToLowerInvariant()

[chriscameron-vertexinc]

I'm having a lot of trouble with this one. The mixing of sync and async is resulting in casting issues.

[chriscameron-vertexinc]

@Lanayx I've been able to drop these changes into my application and verify that the base64 credentials decode is working.

[DanBiscotti]

Hi I'm hoping to use this feature in a .NET Framework project using the 2.x branch, but it looks like it hasn't been merged into that branch. I was able to apply the changes to that branch, although I had to change one line:
match credentialsUrl.LocalPath.Split(',', 2) with -> match credentialsUrl.LocalPath.Split([| ',' |], 2) with
I tested and it seems to work, is it ok to get this change on the 2.x branch as well?

[Lanayx]

Yes, it is ok, you are welcome to send a PR