Refactor _getEtag to get etag from gzip instead of buffer

[JosePineiro]

This PR updates the _getEtag implementation in AsyncWebServerRequest to improve how ETags are generated.

Previously, the method relied on a 4-byte trailer buffer passed as a parameter. This has been replaced with a more robust approach: the function now reads the CRC directly from the last 8 bytes of the gzip file. This ensures that the ETag is derived from the actual file contents rather than depending on an external buffer.

Key changes:

_getEtag now accepts a File handle instead of a trailer array.

The method seeks to the gzip footer (size - 8) and extracts the CRC.

Returns bool to indicate success/failure.

ETag generation logic remains the same, ensuring backward compatibility for clients relying on ETags.

This change improves correctness, reduces external dependencies, and makes ETag generation more consistent with the gzip specification.

[JosePineiro]

This is the same change as in PR #245.
The conflict has been fixed.

[Copilot]

Pull Request Overview

This PR refactors the ETag generation mechanism for gzip-compressed files to improve correctness and robustness. Instead of relying on external trailer buffers, the system now reads the CRC32 checksum directly from the gzip file footer.

• Updated _getEtag method to accept a File handle and return success/failure status
• Modified AsyncFileResponse constructor to use the new ETag generation approach
• Streamlined file opening logic by removing unnecessary else block

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/ESPAsyncWebServer.h	Updated _getEtag method signature to accept File handle and return bool
src/AsyncWebServerRequest.cpp	Implemented new ETag generation logic that reads CRC from gzip footer
src/WebResponses.cpp	Updated AsyncFileResponse constructor to use new ETag method and simplified file opening logic

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