Support upstream TLS certificate trust configuration

A recent Mintlify docs search for TLS skip verify insecure upstream certificate mcpjungle suggests users are trying to connect MCPJungle to HTTPS upstream MCP servers whose certificates are not trusted by the runtime.

Likely cases:

• self-signed local/dev upstream MCP servers
• internal services using a private CA
• corporate TLS interception or private infrastructure certificates

Feature request: support upstream TLS trust configuration for HTTP/SSE MCP server connections. Prefer a safe custom CA option, with an explicitly insecure skip verify escape hatch only if we decide it is necessary for local/dev workflows.

1. Support custom trusted CA certificates for upstream MCP servers. Example config fields:

     {
       "transport": "streamable_http",
       "url": "https://internal.example.com/mcp",
       "tls_ca_file": "/etc/mcpjungle/certs/internal-ca.pem"
     }

2. Optionally support an explicitly dangerous dev escape hatch (should output a strong warning)

     {
       "transport": "streamable_http",
       "url": "https://localhost:8443/mcp",
       "tls_insecure_skip_verify": true
     }

This feature should be documented as applying to MCPJungle -> upstream MCP connections, not client -> MCPJungle gateway TLS.