docs: add design proposal for pluggable message transport interface

[bianbbc87]

What does this PR do / why we need it:
This PR adds comprehensive design documentation introducing a transport abstraction layer for argocd-agent.
The goal is to decouple the principal–agent data stream from the current gRPC dependency and enable multiple transport backends — gRPC, HTTP, and Kafka — for improved compatibility across diverse network environments (e.g., firewalled, air-gapped, or proxy-restricted systems).

This proposal introduces two key abstractions:

• Transport Interface – handles connection management (Connect, Close, CreateConnection)
• Stream Interface – defines event-level data operations (Send, Receive, Close)
  By isolating these layers, the agent can easily switch between different transport protocols without modifying higher-level business logic. This enables a cleaner, more modular architecture for future expansion.

Transport Strategy Notes:
HTTP-based Transport:

• Short/Long Polling (impossible) → too much overhead
• SSE + HTTP POST (possible) → feasible, but event ack is unclear (needs validation)
• WebSocket (possible) → already implemented as fallback, can be extended for use

TCP-based Transport (Message Router Model):

• Introduce Message Router only for queue-based transports
• HTTP transports use direct connections (no router)
• Default router implementation: Kafka (possible)

Key additions:

• Step-by-step communication flow analysis (Before/After)
• Transport interface design and package structure
• Code transformation examples showing minimal changes required
• Implementation strategy for gradual migration

Which issue(s) this PR fixes:

Fixes #260

How to test changes / Special notes to the reviewer:

• This is documentation-only PR, no code changes
• Review the proposed architecture for technical feasibility
• Verify that the Before/After communication flow accurately reflects current implementation
• Check if the package structure reorganization makes sense
• Ensure the migration strategy preserves backward compatibility

Checklist

• Documentation update is required by this PR (and has been updated) OR no documentation update is required.

[daengdaengLee]

remote.go : client.Remote 를 인터페이스로 변환
remote_grpc.go : gRPC 용 client.Remote 구현체
remote_http.go : http 용 client.Remote 구현체
...

agent/ 하위에서는 eventstremapapi 패키지의 인터페이스와 메시지 구조체에만 의존, client.Remote 인터페이스 의존

이런 형태로 pkg/client 쪽 설계가 들어가야 하지 않을까요?

--- english transalted ---

remote.go : convert client.Remote into an interface
remote_grpc.go : gRPC implementation of client.Remote
remote_http.go : HTTP implementation of client.Remote
...

Under the agent/ directory, it only depends on the interfaces and message structs from the eventstreamapi package, and on the client.Remote interface.

Wouldn't this be the right direction for the pkg/client design?

[bianbbc87]

remote.go가 connection 을 담당하고 있어서, 원래 작성했던 connection.go를 제거하고 remote 쪽으로 옮겼습니다 !