Backend Service Provider Interface

[vietj]

Vert.x SQL Client uses a service provider interface (SPI) to communicate with the database.

Currently the SPI is loosely and implicitly defined among internal package : io.vertx.sqlclient.internal shared with other internal classes. This SPI should be isolated in its own package and properly documented.

A TCK should be extracted from the test classes to ensure minimum coverage that can be reused to assess SPI implementation behaviour and conformance.

• decoupling
• documentation
• TCK

[vietj]

cc @yrodiere