Break up db_engine_specs (#7676)

* Refactor db_engine_specs into package

* Rename bigquery class and add epoch funcs

* Fix flake8 errors

* Dynamically load all engine specs

* Fix linting errors and unit tests

* Implement Snowflake epoch time funcs

* Implement Teradata epoch time func

* Fix presto datasource query and remove unused import

* Fix broken datasource query

* Add mypy ignore for false positive

* Add missing license files

* Make create_time_grains_tuple public

* Fix flake8 quote

* Fix incorrect licence header

superset/connectors/sqla/models.py

@@ -38,7 +38,7 @@
 
 from superset import app, db, security_manager
 from superset.connectors.base.models import BaseColumn, BaseDatasource, BaseMetric
-from superset.db_engine_specs import TimestampExpression
+from superset.db_engine_specs.base import TimestampExpression
 from superset.jinja_context import get_template_processor
 from superset.models.annotations import Annotation
 from superset.models.core import Database