Add on-demand sandbox activity protos

[YunchuWang]

The scenario

Durable Task Scheduler needs a shared protobuf contract for sandbox activity workers.

The contract covers two related moments:

1. A client declares that certain activity names should run in a sandbox worker profile, including the container image, provider kind, resource settings, identities, environment variables, and concurrency.
2. A live sandbox worker connects back to DTS, tells the service which declared activities it registered, and sends activity-capacity heartbeats while it processes work.

The .NET and Python preview SDKs both depend on this contract, so the names in the proto become the vocabulary reviewers and SDK users see everywhere else.

What's missing

The first version still used on_demand_sandbox naming and a substrate field name.

That made the contract feel tied to an implementation detail instead of the customer-facing sandbox worker model. It also forced downstream SDKs to either expose the same awkward names or carry translation layers around them.

Review feedback settled on the clearer vocabulary: the service is about sandbox activities, and the provider selector should be named SandboxProviderKind with field name sandbox_provider.

The change

The protobuf surface is renamed around the sandbox worker model.

A good review path is:

1. Start with protos/sandbox_service.proto. This is the source-of-truth contract: package microsoft.durabletask.sandboxes, service SandboxActivities, and enum SandboxProviderKind.
2. Check the request/response messages around declarations. They keep the same product concepts: activity name, worker profile, image, resources, identity, environment variables, and concurrency.
3. Check the worker stream messages. SandboxActivityWorkerStart now carries sandbox_provider, and the rest of the stream still covers worker registration, heartbeat capacity, and responses from DTS.

The old on_demand_sandbox_activities_service.proto surface is replaced by sandbox_service.proto so downstream SDKs can generate and expose the same Sandbox* vocabulary.

[halspang]

Some nits and some questions that may be more about the actual feature instead of this proto directly.