[WIP] support custom metadata in dlt entities and table and column level labelling

[rudolfix]

Background
We want to be able to add additional metadata to dlt objects. The first use is to label schemas so we know if they were created by pipeline, source or by user.

The second use is to label tables and columns pre or during extraction so those labels are available to downstream system that do transformations of dlt generated data.

Labelling of classes should happen via common mixin from which dlt entities should derive. Mixin should allow to accept a json serializable dictionary and expose it as meta property.

Labelling of schema entities should happen via base TypedDict

Which entities

• Pipeline
• destination client configuration (via destination factory)
• DltSource
• DltResource
• Schema
• Tables
• Columns

Tasks

1. • implement Meta mixin
2. • accept meta argument in Pipeline, DltSource, DltResource, Schema and DestinationClientConfiguration (and destination factory when implemented)
3. • accept meta argument in dlt.source and dlt.resource decorators
4. • column and table TypeDicts allow for a "meta" dict with the intention for custom column and table labelling.
5. • implement dlt.mark.label that allows to add table and column level labels to a data item during extraction.