Allow Custom Headers via MCP Client

[rithin-pullela-aws]

Expected Behavior
While constructing the client, we should be able to pass custom headers, which can be used for authorization.
The same functionality is available in the python-sdk

The FlowSseClient and HttpClientSseClientTransport should accept another parameter called headers and set these custom headers when constructing a Http Request. Code

Current Behavior
Currently custom headers are not supported.

Context

[tzolov]

Resolved by eb8e374

[ylwu-amzn]

@tzolov , Thank you so much for the quick turnaround on the PR to fix the issue. Do you have an estimate on when this change will be included in a release?