feat: custom headers support

[omgate234]

Pull Request

Description:
Add support for passing arbitrary custom headers when constructing a Connection, bringing the Node SDK to parity with videodb-python's **kwargs → x-* header behavior.

Changes:

• HttpClient accepts a new optional 3rd argument options?: HttpClientOptions with a headers field; entries are auto-formatted to x-kebab-case and merged into the axios instance defaults alongside x-access-token and x-videodb-client.
• Connection (and the inner VdbHttpClient) forward the new options argument through to HttpClient.
• connect({ ... }) accepts a top-level headers field on ConnectionConfig and forwards it as options.headers to the new Connection signature.
• No new runtime dependencies; legacy connect(apiKey, baseUrl) positional signature is unchanged.

Related Issues:

• Closes #N/A

Checklist:

• Code follows project coding standards
• Tests have been added or updated
• Code Review
• Manual test after merge
• All checks passed