Add support for Azure App Configuration

[Copilot]

Adds an Azure App Configuration builder covering configuration stores, feature flags, and key-value items.

New files

• Arm/ConfigurationStore.fs — ARM types: ConfigurationStore, ConfigFeatureFlag, KeyValue
• Builders/Builders.ConfigurationStore.fs — configurationStore builder with FeatureFlagConfig and KeyValueConfig record types
• Tests/ConfigurationStore.fs — 15 unit tests
• docs/.../app-configuration.md — full documentation page
• Types (Sku, DataPlaneAuthenticationMode) moved to Common.fs per Farmer convention
• Renamed AzFeatureFlag → ConfigFeatureFlag to avoid shadowing Farmer.FeatureFlag
• All optional properties (disableLocalAuth, enablePurgeProtection, publicNetworkAccess, softDeleteRetentionInDays, dataPlaneProxy) default to None rather than being hardcoded
• feature_flags replaced by additive add_feature_flag / add_feature_flags operations
• Added generic key-value support (add_key_value / add_key_values) with optional labels, content type, and per-item tags
• Added Endpoint member returning an ARM expression for the data plane endpoint

I have read the contributing guidelines and have completed the following:

• Tested my code end-to-end against a live Azure subscription.
• Updated the documentation in the docs folder for the affected changes.
• Written unit tests against the modified code that I have made.
• Updated the release notes with a new entry for this PR.
• Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

open Farmer
open Farmer.Builders
open Farmer.ConfigurationStore

let myConfig = configurationStore {
    name "my-app-config"
    sku Standard
    add_tags [ "env", "prod" ]

    add_feature_flags [
        { Name = "DarkMode"; Description = "Enable dark mode"; Label = ""; State = true }
        { Name = "Beta"; Description = "Beta feature"; Label = "beta"; State = false }
    ]

    add_key_values [
        { Key = "Api:BaseUrl"; Label = Some "prod"; Value = "https://api.example.com"; ContentType = None; KeyValueTags = Map.empty }
        { Key = "Settings"; Label = None; Value = """{"timeout":30}"""; ContentType = Some "application/json"; KeyValueTags = Map.empty }
    ]
}

let deployment = arm {
    location Location.NorthEurope
    add_resource myConfig
}

[ninjarobot]

Reviewed and tested the end to end functionality.