feat: add support for PostgreSQL application_name in DSN

[myalcin81]

Description
PostgreSQL has application_name support and its implemented.

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]

You can configure it as:

    public array $default = [
        'DSN'=> "host=localhost port=5432 user=postgres password='postgres' dbname=test options='--application_name=CI4'",
        // ...
    ]; 

So this is an enhancement. So you should send this PR to 4.6 branch.
See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/workflow.md#if-you-sent-to-the-wrong-branch

[kenjis]

This array is settings for MySQLi. So do not add unrelated config item.
Add in

CodeIgniter4/app/Config/Database.php

Lines 75 to 100 in 902ec98

	// /**
	// * Sample database connection for Postgre.
	// *
	// * @var array<string, mixed>
	// */
	// public array $default = [
	// 'DSN' => '',
	// 'hostname' => 'localhost',
	// 'username' => 'root',
	// 'password' => 'root',
	// 'database' => 'ci4',
	// 'schema' => 'public',
	// 'DBDriver' => 'Postgre',
	// 'DBPrefix' => '',
	// 'pConnect' => false,
	// 'DBDebug' => true,
	// 'charset' => 'utf8',
	// 'swapPre' => '',
	// 'failover' => [],
	// 'port' => 5432,
	// 'dateFormat' => [
	// 'date' => 'Y-m-d',
	// 'datetime' => 'Y-m-d H:i:s',
	// 'time' => 'H:i:s',
	// ],
	// ];

[kenjis]

This property is only for Postgre.
Don't add unrelated property in BaseConnection.
Move it to the Postgre class.

[kenjis]

Why is this an array? It seems to be a string.
In any case, add the type for new properties.

[kenjis]

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