fix: features = ["mysql"] not working (#82)

The problem with issue #82 is that when the "mysql" feature is enabled, the default feature is not disabled, resulting in both the "mysql" and "postgres" features being present, causing the code to throw an error of multiple variable definitions. After this modification is applied, when enabling the "mysql" feature, it is necessary to disable the default feature.

When using the "mysql" feature, add the following code to Cargo.toml.

sqlx-adapter = {version = "***", default-features = false, features = ["mysql"]}
tokio = { version = "1.1.1", features = ["macros"] }