[FEATURE]: add @default directive to GraphQL schema

[rderbier]

Allow user to set a default value on fields for add and update operation. Allow use of $now to set current date time as default.

Was covered in v21.12 https://github.com/dgraph-io/dgraph/pull/8017

default value provided as string. $now replaced by current DateTime on the server. See PR, investigate if we can add default value for enums.