Restructure client.Description objects to correctly split Schema and Collections

[AndrewSisley]

Proposed structure:

type CollectionDescription struct {
	Name string
	ID uint32
        GlobalID string // * ID to uniquely identify the collection on a network
	Description string // *
	SchemaVersionID string // **
	Fields []CollectionFieldDescription
	Indexes []IndexDescription
}
type CollectionFieldDescription struct {
	Name string
	ID FieldID // note: If all nodes are on exactly the same Defra version, this would be the same across all nodes, but it does not have to be - make sure this is documented
	Description string // *
	CollectionID uint32 // * for relations
	RelationName string // for relations
	RelationType RelationType // for relations, this type will be modified from the existing definition, it now only needs to say whether it is a one-one or a one-many
}
type SchemaDescription struct {
	SchemaID string
	Name string // note: this is just a label, // thought (from Fred/Keenan): This should probably not actually be defined here, and instead should be a local only thing, otherwise we increase the risk of clashes 
	VersionID string // thought (from Fred/Keenan):  This should probably just be `ID`, we could then rename this struct to `SchemaVersionDescription`
	VersionLabel string // *  // thought (from Fred/Keenan): This should probably not actually be defined here, and instead should be a local only thing, otherwise we increase the risk of clashes 
	Fields []SchemaFieldDescription
}
type SchemaFieldDescription struct {
	Name string
	Kind FieldKind
	CRDT CRDT // **
	SchemaID string // for relations
}
// * == new property
// ** == renamed

Additionally, PatchSchema will remain conceptually as-is, updating the schema object only - PatchCollection will be introduced to do the same for collection stuff. SetDefaultSchemaVersion will be removed, as it's functionality will be covered by PatchCollection.

Worth noting perhaps is that a SchemaFieldDescription will only ever exist for the primary side of a relation. CollectionFieldDescription will exist for both sides.

The pointer from CollectionDescription to SchemaDescription has been removed. I think this makes the separation and [lack of] ownership clearer, as well as making stuff like PatchSchema/Collection more obvious to the user. It may also force us to allow multiple collections from a single schema for free, as the data structure now demands it.

Tasks

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1. Allow labelling of schema versions #1886
   area/query area/schema feature +3
2. Add user-definable description property to collection #1386
   area/query area/schema feature +3
3. Add user-definable description property to fields #1385
   +0
4. Remove Schema from within CollectionDescription #1913
   5 of 5
   area/collections refactor +2
   
5. Rename Schema.SchemaID to Schema.Root #2004
   area/schema refactor +2