Add new connection setting for delay between reconnect attempts

[Namoshek]

As discussed in #129, the auto-reconnect feature only offers basic functionality as of right now. This PR therefore adds an additional connection setting for the delay between reconnect attempts setDelayBetweenReconnectAttempts($delay) which can be used in combination with setReconnectAutomatically(true) and setMaxReconnectAttempts($attempts):

$connectionSettings = (new \PhpMqtt\Client\ConnectionSettings)
    ->setReconnectAutomatically(true)
    ->setMaxReconnectAttempts(3)
    ->setDelayBetweenReconnectAttempts(0);

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✔️ For backwards compatibility, the default delay between reconnect attempts is zero.
✔️ Integration tests for the auto-reconnect feature do not exist and are almost impossible to write, therefore this PR does not contain any tests. Manual tests have been performed.

⚠️ The setDelayBetweenReconnectAttempts($delay) accepts a $delay in milliseconds, which is different to other (timeout) settings.
⚠️ Whether setting a delay between reconnect attempts makes sense, is up to the user of this library. In some scenarios, there is already some kind of delay given in the form of the connect timeout.

[sonarqubecloud]

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27.3% Coverage
0.0% Duplication