Configurable authentication middleware

[kostrse]

The signature of azhttpclient.AzureMiddleware was changed to support configuration via the azhttpclient.AuthOptions options bag. This gives flexibility of introducing new (optional) configuration options and features into the middleware without having to change the function signatures.

One added feature is ability to register custom token providers for custom credential types, other than known by the SDK.

// Initialize the options
authOpts := azhttpclient.NewAuthOptions(azureSettings)

// Configure instance-level scopes
authOpts.Scopes(new []string {"https://datasource.example.org/.default"})

// Optionally, register custom token providers
authOpts.AddTokenProvider("custom-auth-type, func (...) (aztokenprovider.AzureTokenProvider, error) {
})

// Configure client
clientOpts := httpclient.Options{}
azhttpclient.AddAzureAuthentication(&clientOpts, authOpts, credentials)

httpClient, err := httpclient.NewProvider().New(clientOpts)

Fixes #16

[aangelisc]

Once you're ready to change the status of this PR @kostrse then it should be ready for approval. My only request is to update the CHANGELOG to make a note of the breaking change. I realise we haven't been tracking releases properly but we can retrospectively update this.

[aangelisc]

Nice work on this @kostrse 😊