Add support for [Header] activation attribute

[pranavkm]

Similar to #798, with the intent that the data is read from Http headers.

An overload for this would take a string value that specifies the header to bind from:

[Header]
public string Accept { get; set; }

[Header("X-Version")]
public Version ApiVersion { get; set; }

Maybe some smarts: (?)

[Header]
public MimeType ContentType { get; set; }

[danroth27]

@harshgMSFT We should handle this as part of #801.