Define and implement the supported async S7CommPlus authentication boundary

[gijzelaerr]

Problem

AsyncClient currently resembles the synchronous TLS path but lacks V1 SessionKey authentication, legacy HMAC framing, session activation, post-auth legitimation, key renewal and a password argument on connect().

Package documentation broadly claims V1/V2/V3 support without making this async limitation clear. Maintaining two copied protocol state machines is also causing correctness drift.

Scope

First decide and document one explicit support boundary:

1. Bring async legacy authentication to parity, preferably by sharing protocol codecs/state transitions with the sync connection; or
2. Deliberately support TLS-only async operation and fail early with a clear error on legacy SessionKey PLCs.

In either case:

• Share authentication/frame parsing logic where practical.
• Align connection/session-ready semantics with the sync client.
• Add an explicit compatibility matrix to API documentation.
• Prevent unsupported firmware from yielding a misleading connected client.

Acceptance criteria

• Async behavior for V1, V2 and V3 is explicitly documented and enforced.
• Supported paths have end-to-end setup/authentication tests.
• Unsupported paths fail during connect with an actionable message.
• Common framing and authentication rules are not independently reimplemented without parity tests.
• Future sync authentication fixes have an obvious shared or mirrored async test location.