Add defaultDatabase property in Model Protocol

[nick6969]

Description

If there are multiple databases at the same time,
Any Query must specify database,
Cannot use Datebase.dafault directly.

// if not using Datebase.dafault
struct Amodel: Model { }

// every time using find / findAll / save / update .... need put using parameter
// even always same property
Amodel.find(id: 1, using: Database.abc) { model, error in
     // do somrthing
}

But
Model is mean the record struct in the specific table and specific database.
We allow Model specify table,
Why not let Model specify Database?

[CLAassistant]

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