Add a new pgroll baseline command to pgroll that allows users to establish a baseline migration from an existing database schema.

Usage

Run:

$ pgroll baseline 01_initial_version migrations/

This does two things:

• Creates a new entry in pgroll.migrations with a new migration type of baseline. The migration JSON is empty and the resulting_schema field contains the schema at the time the command was run.
• Creates a placeholder migrations/01_initial_version.yaml file for the user to populate (likely with the use of pg_dump).

Features

• New pgroll baseline <version> <target directory> command (currently hidden)
• Creates a baseline migration in the pgroll.migrations table that captures the current schema state without applying changes.
• Validation to prevent baseline creation during active migrations
• Adds test coverage for baseline functionality

Implementation Details

• Added Roll.CreateBaseline method and corresponding tests
• Added State.CreateBaseline method for writing baseline migrations to pgroll.migrations
• Extended migration type definition to include 'baseline' type

The pgroll baseline command is currently hidden. Un-hiding the command will happen when the pull and migrate commands are made baseline-aware.

This is the first part of #364