feat: add support for pinning channels

[akupila]

CLA

• [ ] I have signed the Stream CLA (required).
• [ ] Code changes are tested

Description of the changes, What, Why and How?

This adds support for pinning and unpinning channels. The channel pin is stored on the user-channel relationship (channel membership). The backend API endpoint for this is not yet live.

Changelog

• Add support for pinning channels using updateMemberPartial
• Add channel.pin and channel.unpin
• Fix partialUpdateMember returning a type without the channel_member key that's returned from the API