Added Getter for Table Alias

[dmitriy-malyar89]

Adds the flexibility of using aliases.
Example Query:

public function active()
{
    return $this->andWhere(['=', "$this->alias.status", 'active']);
}

User::find()->alias('instructor')->active()->all();

Result:

SELECT * FROM user instructor WHERE instructor.status = 'active';

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌
Tests pass?	❌
Fixed issues	comma-separated list of tickets # fixed by the PR, if any

[dmitriy-malyar89]

Этот метод позволяет более удобно использовать псевдонимы и гибко использовать их в ActiveQuery.

[samdark]

@yiisoft/reviewers what do you think?

[dmytro-maliar]

Will this request be accepted?

[bizley]

Is calling $this->alias when alias is not set before, setting alias to be default? This sounds weird. And what about calling it first and then chain-calling ->alias('something')?