Add-apikey

[MartinPankraz]

Purpose

Introduces API Key handling and solves #21

Does this solve an existing issue?

[x] Yes - Please add the reference: 
[ ] No

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]

• Test the code

Use API Key protected OData service
maintain API Key and respective header name in appsettings.json
verify additional header gets added for instance checking azure-sdk-for-sap-odata\Dependencies\DataOperations.OData\Client\ODataHttpClientContext.cs

What to Check

• request authenticated with API key and potentially additional auth scheme like Basic Auth

[WillEastbury]

The API key looks a little unorthodox in terms of the interface usage, but it's ok enough to get out of the door for me, feels like in time we should split the API and Auth Headers or change the design into an IAuthHeaderAddingHandler that takes a set of parameters to inject to the request and / or a lambda or delegate to perform the fetch