Add overloads for upload and uploadLink to support custom HTTP headers

[junyeon-aivn]

Changes

• Added overloaded upload() and uploadLink() methods in SecomClient to allow injecting custom headers
• Useful for authentication and tracking metadata via HTTP headers
• No breaking changes; existing methods remain untouched

Why

The original SecomClient design does not expose a way to pass headers. This makes it difficult to use with services requiring token headers or custom context propagation.

Tests

• Manually tested with external integration that requires Authorization headers

[nvasta]

Thank you for the pull-request, quite a good addition I think.

There two points however I would like to raise.

1. First, we now have an upload and uploadLink functionality duplication. I'd suggest we make the non-headers call re-use the one with the headers, and just not pass anything.
2. Is there a method we could use to support additional headers for all calls? For example, add some headers in the client itself (either on initialisation, or a separate function), which can then be included in all WebClient calls. Would that fit your use case?