Maestro-configuration should not require no-pr if PR is already created

[mmitche]

If a specific configuration branch is used for two subsequent commands and a PR for that branch->target already exists, the second command will fail to create a PR. Instead, it should detect whether a PR exists and avoid it automatically.

fail: Failed to create subscription.
      Microsoft.VisualStudio.Services.Common.VssServiceException: TF401179: An active pull request for the source and target branch already exists.
         at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.HandleResponseAsync(HttpResponseMessage response, CancellationToken cancellationToken)
         at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.SendAsync(HttpRequestMessage message, HttpCompletionOption completionOption, Object userState, CancellationToken cancellationToken)
         at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.SendAsync[T](HttpRequestMessage message, Object userState, CancellationToken cancellationToken)
         at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.SendAsync[T](HttpMethod method, IEnumerable`1 additionalHeaders, Guid locationId, Object routeValues, ApiResourceVersion version, HttpContent content, IEnumerable`1 queryParameters, Object userState, CancellationToken cancellationToken)
         at Microsoft.DotNet.DarcLib.AzureDevOpsClient.CreatePullRequestAsync(String repoUri, PullRequest pullRequest) in /_/src/Microsoft.DotNet.Darc/DarcLib/AzureDevOpsClient.cs:line 355
         at Microsoft.DotNet.DarcLib.ConfigurationRepository.RemoteGitRepo.CreatePullRequestAsync(String repositoryUri, String headBranch, String baseBranch, String prTitle, String prDescription) in /_/src/Microsoft.DotNet.Darc/DarcLib/ConfigurationRepository/RemoteGitRepo.cs:line 36
         at Microsoft.DotNet.MaestroConfiguration.Client.ConfigurationRepositoryManager.CreatePullRequest(IGitRepo gitRepo, String repositoryUri, String headBranch, String targetBranch, String title, String description)
         at Microsoft.DotNet.MaestroConfiguration.Client.ConfigurationRepositoryManager.PerformConfigurationRepositoryOperationInternal[TModel](ConfigurationRepositoryOperationParameters parameters, TModel yamlModel, Func`5 operation, String noPrOperationMessage, String prTitle)
         at Microsoft.DotNet.MaestroConfiguration.Client.ConfigurationRepositoryManager.AddSubscriptionAsync(ConfigurationRepositoryOperationParameters parameters, SubscriptionYamlParameters subscriptionParameters)
         at Microsoft.DotNet.Darc.Operations.AddSubscriptionOperation.ExecuteAsync() in /_/src/Microsoft.DotNet.Darc/Darc/Operations/AddSubscriptionOperation.cs:line 421