CollectionDescription Source refactor

[AndrewSisley]

Long term we wish to have something like the below. This issue begins moving us to that.

type CollectionDescription struct {
    // immutable
    ID uint32

    // immutable once set
    GlobalID immutable.Option[string]

    // These properties become immutable if/when GlobalID is set
    Sources         []any
    SchemaVersionID string
    MergeMethod     MergeMethod

    // Always mutable
    Name        immutable.Option[string]
    Indexes     []IndexDescription
    CacheMethod CacheMethod
}

type BaseSource struct {
    ID        uint32  // Used to ID this source, and used to link cached docs to the source (required for a bunch of reasons including update of cache)
    Name      string
    Transform immutable.Option[model.Lens]
}

type QuerySource struct {
    BaseSource
    BaseQuery request.Select
}

type CollectionSource struct {
    BaseSource
    SourceCollectionID uint32
}

type DataStoreSource struct {
    BaseSource
}

type MergeMethod uint32

const (
    MERGE_METHOD_NONE MergeMethod = 0
    MERGE_METHOD_LWW  MergeMethod = 1 //* example only
    // etc
)

type CacheMethod uint32

const (
    CACHE_METHOD_NONE     CacheMethod = 0
    CACHE_METHOD_PER_ITEM CacheMethod = 1
    // etc
)