Add `includes` to individual Meltano yaml files to ref clean Singer yaml

For each tap and target we will create a syntax that references the clean Singer yaml spec for every tap and target. On the YAML we'll have the overrides / additions for each setting and other Meltano-specific items that discovery.yml cares about.

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As the spec is defined for Singer Taps and Targets for the Hub and for the SDK, we'll have to translate from those specs into what's required and useful for Meltano.

For example, one of the key things we document in target-bigquery is how the load_schema is set based on their configuration in the extractor. https://meltano.com/plugins/loaders/bigquery.html#dataset-id That information shouldn't be on the Hub, but it will need to be synced with the hub to work well with Meltano.

2021-05-04 - We're bumping this up in priority to make the sync the clean yaml spec in the hub easier for Meltano. We'll have an includes: syntax and then Meltano-specific things to make the sync easier.