chore: enhance configuration of APISet

[warber]

This PR enhances the configuration of an APISet.
Previously the constructor method for an APISet had quite some arguments which all needed to be passed by the user:

apiutils.NewAPISet(baseURL, authToken, authHeader, client, scheme)

Now, the only required argument to pass is baseURL. Additionally the constructor method was renamed to just New()

apiutils.New(baseURL)

The equivalent way of creating a APISet like above would be:

apiutils.New(baseURL, WithAuthToken(authToken),WithClient(client))

❗ Note that this API is not yet released and is only used in CLI so far, for which a follow-up PR will be made.

[sonarqubecloud]

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[warber]

PR for adaptations in the CLI: keptn/keptn#6655