Breaking Change: Refactor agent constructors to take proxy URI

[blowdart]

In order to flow proxy settings down to the OAuth helper classes it is necessary to change the agent constructors to accept the proxy settings, and other HttpClient settings, rather than take a preconfigured HttpClient.

The new constructor signature will be

protected Agent(
    Uri? proxyUri = null, 
    bool checkCertificateRevocationList = true, 
    string? httpUserAgent = null, 
    TimeSpan? timeout = null)

Previously the constructor signature was

protected Agent(HttpClient? httpClient = null)

This change also removes the utility methods for create a pre-configured HttpClient

• public static HttpClient CreateConfiguredHttpClient(string httpUserAgent, TimeSpan? timeout = null)
• public static HttpClient CreateConfiguredHttpClient(Uri proxyUri, TimeSpan? timeout = null)
• public static HttpClient CreateConfiguredHttpClient(Uri proxyUri, string httpUserAgent, TimeSpan timeout)