Adds support to set charset/collation in config.yml

[douglas]

This allows us to use utf8mb4 charset and utf8mb4_unicode_ci collation in config.yml, which is needed to migrate text that contains emojis.

config.yml example:

mysql:
  database: mysql_database
  username: root
  password: root
  host: 127.0.0.1
  port: 3306
  charset: utf8mb4
  collation: utf8mb4_unicode_ci

postgresql:
  database: pg_database
  username: postgres
  host: 127.0.0.1
  port: 5432
  ssl_mode: disable

[cf-gitbot]

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