Updated DependencyInjection.cs to change ISmsSender and
IEnvelopeSmsHandler lifetimes from Singleton to Scoped,
ensuring per-request instantiation. Added Microsoft.Extensions.Http
package to EnvelopeGenerator.Server.Client.csproj for enhanced
HttpClient handling. Refactored AnnotationService, AuthService,
DocumentService, EnvelopeReceiverService, SignatureCacheService,
and SignatureService to use IHttpClientFactory, improving
flexibility and testability. Introduced a named HttpClient
"EnvelopeGenerator.Server" in Program.cs for internal API calls,
and removed the previous HttpClient setup using HttpContextAccessor.
Added necessary using directives for System.Net.Http across
service files to support these changes.