XATTRS Access Grants

Sync Gateway channels are used partitioning the dataset and for granting users access to specific documents via channel assignment.  Currently, the channels that a document is assigned to has to be either derived from the contents of the document or explicitly specified via a special “channels” property in the document. The enhancement proposed in this document is to allow users to leverage user-defined [Extended Attributes(XATTRs)|https://docs.couchbase.com/java-sdk/current/concept-docs/xattr.html] for storing access grants and to keep that separate from the document body.

PRD:  https://docs.google.com/document/d/1bTxq83PFfKFVIwPlziOv_FhG3eik31NzygRobzrxR_c/edit?ts=602308d7#heading=h.262na8yxv5kl 

Design: https://docs.google.com/document/d/1X1_qqNNZIox8yjQMeCvqKx_AC7_Gau1EAAHVCg1Sl_0/edit#heading=h.qy9u9vld6pz6 

Impacted Assets:

• Whats New (done - 4/20/21)
• Release notes - new feature – (done 4/20/21)
• Release notes - changed behavior 
• sync-gateway-channels.adoc
• defining-sync-functions.adoc
• sg.yaml
• sync-gateway-admin.yaml (_raw)

Dev Tickets: 

• Tracking — https://issues.couchbase.com/browse/CM-354

Docs Tickets:

• Prep — https://issues.couchbase.com/browse/DOC-8177
• Epic — https://issues.couchbase.com/browse/DOC-8065