Support "--header" for invoke CLI API

[denismakogon]

Taking into account that Invoke API doesn't support query parameters there are only two ways to pass configuration/secrets to a function:

• headers
• config

For those cases when you don't want to expose your secrets via config options the only option that is left is headers (which is more flexible because header-way makes a function flexible enough obviously).

Would be "nice to have" to enable headers through --header for fn invoke