Add webhook verification support

[bpapillon]

No description provided.

[Copilot]

Pull Request Overview

This PR adds support for verifying webhook signatures via HMAC-SHA256, including implementation of the verifier and accompanying tests, a simple test server, and updated documentation.

• Added WebhookVerifier and WebhookSignatureException for signature verification
• Implemented unit tests in WebhookVerifierTest and an HTTP test server in WebhookTestServer
• Updated README.md with usage instructions for verifying webhook signatures

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/test/java/com/schematic/webhook/WebhookVerifierTest.java	Added tests to ensure correct signature computation and verification
src/main/java/com/schematic/webhook/server/WebhookTestServer.java	Introduced a test server to simulate webhook requests and verify signatures
src/main/java/com/schematic/webhook/WebhookVerifier.java	Implemented webhook signature verification logic using HMAC-SHA256
src/main/java/com/schematic/webhook/WebhookSignatureException.java	Created a custom exception for signature verification failures
README.md	Updated documentation with instructions for webhook signature verification

Files not reviewed (1)

• .fernignore: Language not supported

[Copilot]

Consider using body.getBytes(StandardCharsets.UTF_8).length instead of body.length() when setting the response header to accurately calculate the number of bytes, especially for non-ASCII characters.

Suggested change

	exchange.sendResponseHeaders(statusCode, body.length());
	exchange.sendResponseHeaders(statusCode, body.getBytes(StandardCharsets.UTF_8).length);

[Copilot]

It would be beneficial to validate that the hex string has an even length before processing, as an odd-length string would indicate an invalid format.

Suggested change

	private static byte[] hexToBytes(String hex) throws WebhookSignatureException {
	private static byte[] hexToBytes(String hex) throws WebhookSignatureException {
	if (hex.length() % 2 != 0) {
	throw new WebhookSignatureException("Invalid hex string format: length must be even");
	}