add IndexedValueType to proto IDL

Use this template for feature, improvement, or cleanup requests.

Description

Add IndexedValueType to proto IDL. Currently, the client has to align with how server deserialize the Payload for search attributes. This requires special domain knowledge to ensure correctness and is error-prone.

A better approach is to add additional IDL for IndexedValueType so it's an agreement on how clients should serialize into Payload.

This is similar to how featureflag is encoded/decoded today.

Is this a breaking change?

• Yes
• No

If yes, explain why:

Backward compatible if IDL is set to be backward compatible.

Scope of the feature (server, specific client, all clients)

Server and all clients