Make the pgroll migrate command baseline-aware. This means that only migrations since the most recent baseline are considered for application.

This PR:

1. Adds a LatestBaseline method to the State package that retrieves the most recent baseline migration for a schema
2. Changes the UnappliedMigrations function to properly handle baselines by only considering migrations after the most recent baseline
3. Adds test coverage for baseline functionality with various scenarios

Example workflow

Initial Setup

Database: Empty
Migration Files: None
Migration History: Empty

Step 1: Create Initial Migrations

# 01_create_users.yaml
name: 01_create_users
operations:
  - create_table:
      name: users
      columns:
        - name: id
          type: serial
          pk: true
        - name: name
          type: varchar

# 02_create_posts.yaml
name: 02_create_posts
operations:
  - create_table:
      name: posts
      columns:
        - name: id
          type: serial
          pk: true
        - name: title
          type: varchar

Run pgroll migrate:

Migration History:
- 01_create_users (pgroll, done=true)
- 02_create_posts (pgroll, done=true)

Step 2: Create a Baseline

$ pgroll baseline 03_baseline

Migration History:
- 01_create_users (pgroll, done=true)
- 02_create_posts (pgroll, done=true)
- 03_baseline (baseline, done=true)

Step 3: Add More Migrations

# 04_add_comments.yaml
name: 04_add_comments
operations:
  - create_table:
      name: comments
      columns:
        - name: id
          type: serial
          pk: true
        - name: content
          type: text

Step 4: Run pgroll migrate Again

When running pgroll migrate, pgroll:

1. Finds latest baseline (03_baseline)
2. Only applies migrations after the baseline

Migration History:
- 01_create_users (pgroll, done=true)
- 02_create_posts (pgroll, done=true)
- 03_baseline (baseline, done=true)
- 04_add_comments (pgroll, done=true)

Part of #364