Dtspec

* Resolves an issue loading Snowflake with empty data

* Resolve issue with cleaning up databases with Snowflake.
* Allows dtspec to compile dbt snapshots as valid refs

* Resolves issue with Snowflake authentication options when using username/password
* Creates test target schema if not exists prior to checking for table/view existence
* Serialize special SQLAlchemy values prior to loading

* Resolve confusing `project_dir` that should have been `profile_dir` CLI argument (mdesmit)
* Instead of raising an error, will drop targets that are modeled as views
* Support for snowflake private key authentication (mdesmit)
* CLI Integration tests using dbt-container-skeleton project
* Using github actions for CI

* New `dtspec` cli command that helps with setting up test environments and running against dbt projects.

* Identifiers can now be embedded in both source and expectations.