Add support for protected and unprotected headers

[msftsettiy]

This PR contains code and tests to allow users to supply protected and un-protected headers in the tool. Two options are provided for specifying headers. They are as follows:

1. A JSON file containing the collection of headers.
2. Command-line flags to specify a collection of name-value pairs that represent the headers.

In this iteration, headers containing either an int32 value or a string value are supported.

Help text for the sign command and CoseSignTool.md are updated.