chore: make channel optional on UploadFileV2

[0xdeafcafe]

Pull Request Guidelines

These are recommendations for pull requests.
They are strictly guidelines to help manage expectations.

PR preparation

Run make pr-prep from the root of the repository to run formatting, linting and tests.

Should this be an issue instead

• is it a convenience method? (no new functionality, streamlines some use case)
• exposes a previously private type, const, method, etc.
• is it application specific (caching, retry logic, rate limiting, etc)
• is it performance related.

API changes

Since API changes have to be maintained they undergo a more detailed review and are more likely to require changes.

This change is non-breaking, and will allow users to upload files using the new v2 endpoint that don't have channel id, so they won't automatically be posted. This is supported by Slack and needed if users want to attach the files to a post vs have them posted in the same thread/channel later.

Examples of API changes that do not meet guidelines:

None.