Return signed URL directly in TikTokSignatureProvider instead of calling sign URL endpoint

[hoangvu12]

The webcast/sign_url endpoint is deprecated, so this PR rewrites the function to return the signed URL directly instead of calling the endpoint. This should theoretically improve the speed since the library will only need to make one request instead of two.

This PR also removes config and signEvents as they are not used elsewhere.

Breaking changes: this might cause issues for people who are using a custom host and the webcast/sign_url endpoint to sign URL. Additionally, this change eliminates the need for signProviderOptions.params and signProviderOptions.headers.