feat(rmcp): Add authorization header support for the streamable http client

[mjcarson]

This allows the streamable http client to send an authorization header when making requests.

Closes#387

This simply allows users to specify an auth header to use with the streamable http client.

// build the config to use with this transportlet config = StreamableHttpClientTransportConfig::with_uri("<MCP_SERVER>")// set our auth header.auth_header("<AUTH_HEADER>");// setup our transportlet transport = StreamableHttpClientTransport::from_config(config);// build our client infolet client_info = ClientInfo{protocol_version:Default::default(),capabilities:ClientCapabilities::default(),client_info:Implementation{name:"Example MCP Client".to_string(),version:"0.0.1".to_string(),},};// build our client
client_info.serve(transport).await.unwrap()
```
## Motivation and ContextPrior to this it was not possible for this client to be used with MCP servers that use the authorization header for auth.
## HowHasThisBeenTested?
Yes,I tested this against a local MCP server.
## BreakingChangesNo, this should be a purely additive feature.
## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
- []Bugfix(non-breaking change which fixes an issue)
- [x]Newfeature(non-breaking change which adds functionality)
- []Breakingchange(fix or feature that would cause existing functionality to change)
- []Documentation update
## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes that apply. -->
- [x]I have read the [MCPDocumentation](https://modelcontextprotocol.io)
- [x]My code follows the repository's style guidelines
- [x]New and existing tests pass locally
- []I have added appropriate error handling
- []I have added or updated documentation asneeded
## Additional context
<!-- Add any other context, implementation notes, or design decisions -->

[4t145]

looks good