YANG Metadata Annotation for Immutable Flag

This document defines a way to formally document an existing behavior, implemented by servers in production, on the immutability of some system-provided nodes, using a YANG metadata annotation called "immutable" to flag which nodes are immutable. Clients may use "immutable" annotations provided by the server, to know beforehand why certain otherwise valid configuration requests will cause the server to return an error. The immutable flag is descriptive, documenting an existing behavior, not proscriptive, dictating server behaviors. This document updates RFC 8040 and RFC 8526.