fix: preserve substitutions from To in Personalization

[toleabivol]

If there are substitutions in the To object preserve them by transfering them to the Personalization object.

Currently this does not work (we loose all substitutions from the To):

$personalization0 = new Personalization();
$personalization0->addTo(new To(
        "[email protected]",
        "Example User2",
        [
            '-name-' => 'Example User 2'   // <--this is lost and not sent to sendgrid API
        ],
        "Example User2 -name-"
));
$email->addPersonalization($personalization0);

We can either add the substitution during addTo method (as current PR is proposing) or parse all Tos during getSubstitutions method and add them to the substitution property. I propose the first way because it is similar to what we have in https://github.com/sendgrid/sendgrid-php/blob/main/lib/mail/Mail.php#L205

We have to also think about implementations that worked around this issue (like myself) and not break them, so the following must still be working :

$personalization0 = new Personalization();
personalization->addSubstitution(new Substitution('-name-', 'Example User 1')); // <- we want this preserved even if added first
$personalization0->addTo(new To(
        "[email protected]",
        "Example User2",
        [
            '-name-' => 'Example User 2' // <- we want this to not be applied in this case as addSubstitution takes precedence
        ],
        "Example User2 -name-"
));

$email->addPersonalization($personalization0);

OR we don't but we risk to break some implementations. Are there means of communicating the change in this case ?

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