Use templating engine instead of plain yaml

[redblackcoder]

The plain yaml spec lacks many validations and other flexibilities that a strong templating config templating language like Jsonnet or CUE can provide.

Few issues that are present in the current spec in plain yaml are:

1. The schema for various elements in the spec are not enforable, even though they are specified in the yaml. E.g. dialects enum is present but cannot be enforced.
2. Many constraints cannot be enforced, like primary_key should be one of the fields, the attributes in the relationship should be valid, etc.
3. The structure for various attributes fields, relationships, etc. are not enforcable, and when multiple attributes need to share the same struct, it is not possible in plain yaml.