SDK transport interface: decouple from gRPC

[zeevdr]

Description

Introduce a transport interface in the SDK so consumers aren't forced into a specific gRPC version. The SDK currently takes grpc.ClientConnInterface directly, coupling consumers to our pinned gRPC version (v1.79.3, requires Go 1.24).

Approach

Define a transport interface per SDK module:

type Transport interface {
    GetConfig(ctx context.Context, req *GetConfigRequest) (*GetConfigResponse, error)
    // ...
}

type GRPCTransport struct { conn grpc.ClientConnInterface }

gRPC becomes an implementation detail in a separate sub-module (sdk/grpctransport or similar). Consumers can bring their own gRPC version or a non-gRPC transport.

Module structure after this change

sdk/configclient/     → go 1.22, no gRPC dependency
sdk/adminclient/      → go 1.22, no gRPC dependency
sdk/configwatcher/    → go 1.22, depends on configclient interface
sdk/grpctransport/    → go 1.24+, depends on gRPC, implements transport interfaces
sdk/tools/            → unchanged

Acceptance criteria

• Transport interface defined in configclient and adminclient
• GRPCTransport as default implementation in sdk/grpctransport
• Existing New(conn) convenience API preserved (in grpctransport)
• configwatcher uses configclient's interface, not gRPC directly
• All tests pass
• Migration guide in SDK README

Notes

• Phase 2 of SDK dependency reduction (Phase 1: Remove testify from SDK modules #159 removes testify)
• Enables Phase 3: Lower SDK minimum Go version after transport decoupling #161 lowers Go minimum to 1.22
• Consider whether protobuf types in the interface are acceptable or need their own abstraction