Support custom HTTP headers for MCP transport

[quaff]

Closes GH-3948

[tzolov]

@quaff hopefully this case is addressed by #2506 (comment) ?

[quaff]

@quaff hopefully this case is addressed by #2506 (comment) ?

@tzolov I don't think so, it's a different issue, this PR aims to pass long-live access token, and other custom headers.

[ilayaperumalg]

@Kehrlann Please share your review on this. Thanks!

[Kehrlann]

After discussion with the team, we have decided not to expand the configuration surface.

If you would like to add headers for which you know the value in advance, please use the following java configuration:

1. For web-client based support, provide your own WebClient.Builder:

@Bean
WebClient.Builder mcpClient() {
    return WebClient.builder().defaultHeader("x-custom-header", "<HEADER-VALUE>");
}

2. For HTTP-client based, provide McpSyncRequestCustomizer or McpAsyncRequestCustomizer:

@Bean
McpSyncHttpClientRequestCustomizer requestCustomizer() {
    return (builder, method, endpoint, body, context) -> builder.header("x-custom-header", "<HEADER-VALUE>");
}