add support for overlays in kiota

OpenAPI overlays allow people to maintain patches to an OpenAPI description in a separate document.
This is especially useful when the person generating a client is not the one authoring the OpenAPI description, and they don't want to maintain in sync their own modified copy.
This topic has come in discussions times and times again see: #6687 #6451 #5073 #3406 #411 and many more.

To implement overlays we first need to:

declare a new option

Name "overlay".
Supports multiple values.
Defaults to an empty array/list.
full name --overlay
Short name --ov

https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/KiotaHost.cs#L427

use that option in commands

• generate command https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/KiotaHost.cs#L436
• add command for clients https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/KiotaClientCommands.cs#L32
• edit command for clients https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/KiotaClientCommands.cs#L107
• edit command for plugins https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/KiotaPluginCommands.cs#L120
• add command for plugins https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/KiotaPluginCommands.cs#L73

pass the option to handlers

Example with generate

• https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/Handlers/KiotaGenerateCommandHandler.cs#L64
• https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/Handlers/KiotaGenerateCommandHandler.cs#L101
• https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/Handlers/KiotaGenerateCommandHandler.cs#L135

(do the same with client/plugin add/edit commands)

Add the option to the generation configuration

https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/Kiota.Builder/Configuration/GenerationConfiguration.cs#L117

Add the option to the RPC server

https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/kiota/Rpc/Server.cs#L156

(same for plugins)

add the option in the TypeScript API

https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/vscode/packages/npm-package/lib/generateClient.ts#L57

(same for plugins)

add the dependency to the csproj

Add BinkyLabs.OpenAPI.Overlays to src/kiota.builder/kiota.builder.csproj

Intercept the loading and apply the overlays

https://github.com/microsoft/kiota/blob/0772a6e5746ae8a264fcda6626a7d667bbf55916/src/Kiota.Builder/OpenApiDocumentDownloadService.cs#L140

Add some unit tests

https://github.com/microsoft/kiota/blob/main/tests/Kiota.Builder.Tests/OpenApiDocumentDownloadServiceTests.cs

store the overlays with the workspace service (caching)

https://github.com/microsoft/kiota/blob/main/src/Kiota.Builder/WorkspaceManagement/WorkspaceManagementService.cs

Start with the option definitions and command entry points in src/kiota/KiotaHost.cs, KiotaClientCommands.cs, and KiotaPluginCommands.cs, then trace the generate handler into GenerationConfiguration.cs. Review OpenApiDocumentDownloadService.cs and WorkspaceManagementService.cs, along with the RPC server and TypeScript API entry points. Done means overlays are accepted across the listed commands and APIs, applied during document loading, cached, and covered by OpenApiDocumentDownloadServiceTests.cs.