Allow overriding Bearer/Digest Authorization tags

[Badgerati]

Describe the Change

Like with Basic authentication, the tag in the Authorization header can be changed. For Bearer and Digest authentication, this isn't the case, and it sometimes you might need Bearer to be "api-key" instead.

Therefore, on New-PodeAuthScheme the -HeaderTag should be extended to Bearer and Digest - and it's default set to blank (this default does nothing anyway!).