ECS support infra - Introducing ECS feature flag read call via a common module

[tyaginidhi]

This module can be consumed for reading the latest feature flags set for a user in both desktop and web extension. The API call will currently honor userId, tenantId, region and tenantId filters. More filters support will be introduced in subsequent PR.

Covered:
Why direct ECS API consumption here: ECS JS library is currently not supported for external repo - so this integration has a go ahead to directly consume the ECS API.

NOTE: AI generated details below this line:

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This pull request introduces a new feature configuration system for the project, including the addition of a new ECSFeaturesClient class and related utilities. The most significant changes involve the creation of a set of new files in the src/common/ecs-features directory, each of which contributes to the overall functionality of the feature configuration system. Additionally, a minor change was made to src/web/client/utilities/commonUtil.ts.

Here is a detailed summary of the changes:

New Constants and Types:

• src/common/ecs-features/constants.ts: Added constants related to the ECS feature system, such as ECS_REQUEST_URL_TEMPLATE and PowerPagesClientName.

New Client for ECS Features:

• src/common/ecs-features/ecsFeatureClient.ts: Introduced the ECSFeaturesClient class, which is responsible for initializing and getting the configuration of ECS features.

New Interfaces for ECS Features:

• src/common/ecs-features/ecsFeatureFlagFilters.ts: Added the ECSAPIFeatureFlagFilters interface, which represents the filters used for ECS API feature flags.
• src/common/ecs-features/ecsFeatureProperties.ts: Introduced the ECSFeatureDefinition and ECSFeatureInfo interfaces, which represent the properties of an ECS feature. Also added a function createECSFeatureDefinition to create a feature object based on the feature definition.

New Utilities for ECS Features:

• src/common/ecs-features/ecsFeatureUtil.ts: Added utility functions like getECSRequestURL and powerPagesFeatureClient to support the ECS feature system.

Feature Gates:

• src/common/ecs-features/ecsFeatureGates.ts: Defined the EnableMultifileVscodeWeb feature and its fallback configuration.

Minor Change:

• src/web/client/utilities/commonUtil.ts: A line break was added after the getBackToStudioURL function.

the license text update is out of scope for this PR - rest of the comments are addressed