feat(x): add ECH GREASE support to fetch tool

[fortuna]

Not working yet. For some reason I'm getting server rejections. Not sure what I'm doing wrong.

% go -C x run ./tools/fetch -v --ech-config=grease https://example.com
Sep  2 11:21:48.572 ERR HTTP request failed error="Get \"https://example.com\": tls: server rejected ECH"
exit status 1

Error source:
https://cs.opensource.google/search?q=%22server%20rejected%20ECH%22&ss=go%2Fgo

Note that with a valid ECH Config from a different site it does the handshake (though the certificate validation fails as expected):

% go -C x run ./tools/fetch -v --ech-config=AEb+DQBCqQAgACBlm7cfDx/gKuUAwRTe+Y9MExbIyuLpLcgTORIdi69uewAEAAEAAQATcHVibGljLnRlc3QuZGVmby5pZQAA https://example.com
Sep  2 11:22:24.446 ERR HTTP request failed error="Get \"https://example.com\": tls: failed to verify certificate: x509: certificate is valid for a248.e.akamai.net, *.akamaized.net, *.akamaized-staging.net, *.akamaihd.net, *.akamaihd-staging.net, not public.test.defo.ie"
exit status 1

[Copilot]

Pull Request Overview

This PR adds support for ECH (Encrypted Client Hello) GREASE functionality to the fetch tool. The implementation allows testing ECH behavior by generating GREASE ECH configurations that can help identify middleboxes that reject ECH-enabled connections.

• Implements ECH GREASE configuration generation using HPKE X25519 key pairs
• Adds command-line support for ECH configuration in the fetch tool
• Includes comprehensive test coverage for the ECH GREASE functionality

Reviewed Changes

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

File	Description
x/tools/fetch/main.go	Adds ECH support with --ech-config flag and GREASE mode
x/ech/ech_grease.go	Implements ECH GREASE configuration list generation
x/ech/ech_grease_test.go	Provides test coverage for ECH GREASE functionality

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[Copilot]

The error message is misleading for the GREASE case since no base64 decoding is performed. Consider using a more accurate message like 'Failed to generate GREASE ECH config'.

Suggested change

	slog.Error("Failed to decode base64 ECH config", "error", err)
	slog.Error("Failed to generate GREASE ECH config", "error", err)

[Copilot]

The magic number 42 for maximum_name_length should be replaced with a named constant or documented inline comment explaining its purpose.