Simplify DI for ManagementClient

[sevcik-martin]

Motivation

Enables user to use it simply with DI

Proposed solution

Can be done using extension methods. For inspiration see kontent-delivery-sdk-net

[pokornyd]

shipped in 9.0.0-beta-1, modeled on the delivery SDK's extensions as proposed here.

 // bind from configuration…
 services.AddManagementClient(configuration.GetSection("ManagementOptions"));
 // …or configure inline
 services.AddManagementClient(options => { options.EnvironmentId = "…"; options.ApiKey = "…"; });

Integrates with IHttpClientFactory, validates options at startup, supports multiple named clients, and exposes hooks for the HTTP pipeline and resilience.

will close this with GA release of v9