feat: add Model field casting

[kenjis]

Needs #8230, #8260, #8539

Description
Entity is optional, and some devs do not want to use it.
See https://forum.codeigniter.com/showthread.php?tid=85956
This PR provides field type casting in Model so that all custom entities (or arrays) can use type casting.
This PR can solve #7177, #5905.

Model Field Casting

This casting only works when data comes from and goes to database.

[Object] --- toDataSource() ---> [DB driver]
[Object] <-- fromDataSource() -- [DB driver]

To use this feature, add property $casts to a Model.

    protected $casts = [
        'id'         => 'int',
        'email'      => 'json-array',
        'created_at' => 'datetime',
        'updated_at' => 'datetime',
    ];

Entity and this feature

Entity casting works at (1)(4), but this casting works at (2)(3).

[App Code] --- (1) --> [Entity] --- (2) --> [Database]
[App Code] <-- (4) --- [Entity] <-- (3) --- [Database]

If you use this feature with Entity, do not use casting in Entity.
Using both casting at the same time does not work.

When using casting in Model, Entity has correct typed PHP values in the attributes.
This behavior is completely different from the previous behavior.
Do not expect the attributes hold raw data from database.

TODO

• find*()
• insert()
• update()
• save()
• don't break $useTimestamps
• don't break Entity

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Implementation of this feature is complete.
I believe you can use casts in almost any class (custom entity) that is not an Entity.

[kenjis]

Updated to use $db->dateFormat and $updateOnlyChanged.