Add user-definable description property to fields

[AndrewSisley]

Add user-definable description property to fields (client.FieldDescription, GQL SDL, JsonPatch), and display the field-level description via the GQL type system (e.g. on the query-fields).

GQL Descriptions in an SDL is defined in the spec: https://spec.graphql.org/October2021/#sec-Descriptions

Note: graphql-go does not support parsing of comments/descriptions from the SDL: https://github.com/graphql-go/graphql/issues/116

Adding them via PatchCollection should be pretty trivial, we can always do that first, then figure out the SDL problem later. We could as a temporary workaround have a @description directive.

Related to: https://github.com/sourcenetwork/defradb/issues/1386