Snowddl

- Implemented first version of `snowddl-singledb` entry-point. It is a simplified version of SnowDDL to manage schemas and objects in a single database only. Account-level objects, roles and grants are NOT resolved in this mode. Please check the documentation for more details.
- Schemas will no longer produce `DROP SCHEMA ...` SQL commands during `destroy` action without `--apply-unsafe` flag, similar to schema objects. All schemas are dropped implicitly after execution of `DROP DATABASE` anyway.
- Added `database_full_name` property for `SchemaIdent` and `SchemaObjectIdent` objects to simplify access to corresponding `DatabaseIdent` object.
- Replaced `argparse.Namespace` with basic `dict` for handling of CLI arguments. It helps to streamline access to specific arguments which may not be defined in other entry-points.

- Fix missing grants for `schema_owner`, `schema_write`, `schema_read` business role options without wildcards.

- Speed up SnowDDL execution by loading grants and future grants of existing roles in parallel.

- Added a basic wildcard option while setting `schema_owner`, `schema_write`, `schema_read` options for business roles to match "all schemas in database". For example: `snowddl_db.*`. At least one schema matching wildcard condition should exist in config.

It is useful for managing generic script roles when new schemas are added and / or removed frequently.

- Identifier objects were completely reworked. Now every identifier type has its own class with every part being named.
- Simplified blueprint objects. Removed `database`, `schema`, `name` fields from schema object blueprints. All this information is available as parts of `full_name`.
- Moved complex logic for "building" identifiers into dedicated module `ident_builder`.
- Performed initial preparation and testing for "singledb" entry point, which will be added in the next version.

- Rework internal architecture of entry-points for SnowDDL CLI interface. Now it will be much easier to add new entry-points and to partially re-use existing entry-points in your own code.