Add custom JSON encoder / decoder support

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Adds support for configuring which JSON coders MySQL uses when conforming to SQLKit's APIs (#285, fixes #252).

// Some existing conformer to `MySQLDatabase`
let mysql: MySQLDatabase = ...

// Setup custom JSON coders that use unix timestamps
let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .secondsSince1970
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .secondsSince1970

// Use `sql()` method to create a `SQLDatabase` passing in
// MySQL coders that use the custom JSON coders.
let sql = mysql.sql(
    encoder: MySQLDataEncoder(json: encoder), 
    decoder: MySQLDataDecoder(json: decoder)
)

// Sample model with nested JSON field.
struct Foo: Codable, Equatable {
    struct Bar: Codable, Equatable {
        var baz: Date
    }
    var bar: Bar
}

// New instance to create.
let foo = Foo(
    bar: Bar(
        baz: Date(timeIntervalSince1970: 1337)
    )
)
// INSERT INTO `foo` (`bar`) VALUES (?) ["{\"baz\":1337}"]
try db.insert(into: "foo").model(foo).run().wait()

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These changes are now available in 4.0.0-rc.1.8