feat(esp8266): support MQTT TLS with fingerprint pinning

[2asoft]

Summary

Add ESP8266 MQTT TLS support with server fingerprint pinning.

This wires the MQTT client to the ESP8266 BearSSL-capable ESPAsyncTCP transport so secure connections can be configured with pinned server fingerprints and fingerprint failures surface through the existing
disconnect reasons.

Changes

• add ESP8266 TLS session setup for the BearSSL transport
• plumb configured server fingerprints into the ESP8266 TLS connect path
• map fingerprint mismatches to AsyncMqttClientDisconnectReason::TLS_BAD_FINGERPRINT
• keep the existing non-ESP8266 SSL fingerprint behavior intact
• relax the ESP8266 TCP dependency in library.json from a pinned package identity to the generic ESPAsyncTCP library name, so consuming projects can choose the concrete ESP8266 TCP library source

Why

ESPHome needs secure MQTT on ESP8266 with fingerprint pinning. This restores that capability.

Verification

Verified downstream in ESPHome using local dependency overrides to this branch and the matching ESPAsyncTCP branch.