[FEATURE] Support configurable header passthrough for MCP server requests

[Derek2Tu]

Description:

Currently, ToolHive passes through certain headers to MCP servers as a side effect of not stripping them, rather than through intentional configuration. We should implement explicit support for specifying additional headers to forward to MCP servers.

Requirements:

• Add configuration option to define a list of headers that should be passed through to MCP servers
• Support this functionality in both local and Kubernetes deployment modes
• Make header passthrough behavior intentional and documented rather than incidental

[jhrozek]

@tgrunnagle @glageju do we have more details on this?

[tgrunnagle]

I think this refers to the cloud_id header for Atlassian - if so the existing header pass-through from the client supports it. @Derek2Tu can you confirm?