Add pre and post export setup name env variable settings

[AlexanderVorobyov]

• Add BuildEditorSettingsData.PreExportSetupNameEnvironmentVariable property to define environment variable with setup name for pre export execution.
• Add BuildEditorSettingsData.PostExportSetupNameEnvironmentVariable property to define environment variable with setup name for post export execution.
• Add BuildEditorUtility.ExecutePreExport() and PostExport() methods used to execute setup from environment variables and specified context.
• Change BuildEditorUtility.TryGetSetupNameFromEnvironmentVariables() and GetSetupNameFromEnvironmentVariables() methods to require environment variable name to get setup name from.
• Fix UnityCloudBuild.PreExport() and PostExport() methods create context with Unity Cloud Build manifest included.
• Remove BuildEditorSettingsData.SetupNameEnvironmentVariableName property, use PreExportSetupNameEnvironmentVariable and PostExportSetupNameEnvironmentVariable properties instead.
• Remove BuildEditorUtility.Execute() method with zero arguments.
• Remove UnityCloudBuildEditorUtility.Execute() method.

[AlexanderVorobyov]

Linked Issue: #13.