Set custom Headers for HTTP request

[lipkau]

Expected Behavior

User is able to set custom headers for the HTTP request. This idea is spun off @Dejulia489's comment on #121.

Possible Solution

• create something that sets the headers to the PrivateData of the module or a config file (like how the session or server are stored) Set-JiraHeaders?
• implement -Headers to every single public function
• user uses $PSDefaultParameterValues - already supported

$PSDefaultParameterValues["Invoke-WebRequest:Headers"] = @{ ..... }