Support add custom http header in WebFluxSseClientTransport

[SunJiFengPlus]

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Expected Behavior

I would like to add custom HTTP request headers to WebFluxSseClientTransport

like this, overload sendMessage method

	@Override
	public Mono<Void> sendMessage(JSONRPCMessage message) {
		return sendMessage(message, httpHeaders -> {});
	}

	public Mono<Void> sendMessage(JSONRPCMessage message, Consumer<HttpHeaders> mutateHeaders) {
		// The messageEndpoint is the endpoint URI to send the messages
		// It is provided by the server as part of the endpoint event
		return messageEndpointSink.asMono().flatMap(messageEndpointUri -> {
			if (isClosing) {
				return Mono.empty();
			}
			try {
				String jsonText = this.objectMapper.writeValueAsString(message);
				return webClient.post()
					.uri(messageEndpointUri)
					.headers(mutateHeaders)
					.contentType(MediaType.APPLICATION_JSON)
					.bodyValue(jsonText)
					.retrieve()
					.toBodilessEntity()
					.doOnSuccess(response -> {
						logger.debug("Message sent successfully");
					})
					.doOnError(error -> {
						if (!isClosing) {
							logger.error("Error sending message: {}", error.getMessage());
						}
					});
			}
			catch (IOException e) {
				if (!isClosing) {
					return Mono.error(new RuntimeException("Failed to serialize message", e));
				}
				return Mono.empty();
			}
		}).then(); // TODO: Consider non-200-ok response
	}

Current Behavior

Can't add custom header in WebFluxSseClientTransport

Context

If I can change the HTTP request header, I can do some extra processing on the server side like data permission checking. I briefly checked this article https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization It doesn't seem to have been developed yet, so I don't know exactly how it will be used.

For common API Server to MCP Server conversion scenarios, the API Server already has its own authentication methods, and functional access control, data access control, for the current only missing the HTTP Header transfer.