WIP: create sqlx.toml format

[abonander]

Reference sqlx.toml: https://github.com/launchbadge/sqlx/blob/sqlx-toml/sqlx-core/src/config/reference.toml

Once this is implemented, I'm planning on starting an alpha release cycle for 0.9.0 so that we can test this. It's not expected to be a breaking change but it's a large enough feature that it should probably go in a new major release, and there's a few other breaking change APIs waiting in the wings.

Bikeshedding checklist:

• snake_case or kebab-case for keys?
• macros.column_overrides should maybe be called macros.table_overrides instead?
  • since the next level in the key is the table name, not the column name?

Implementation checklist:

• [common]
  • database_url_var
    • support in query macros
    • support in sqlx-cli
• [macros]
  • datetime_crate
  • column_overrides
  • type_overrides
• [migrate]
  • table_name
  • migrations_dir
  • ignored_chars
  • default_type
  • default_versioning