Allow RocketChatAttachment class accept DateTimeImmutable as timestamp argument

[antonkomarev]

This PR adds failing tests for DateTimeImmutable timestamp. This need to be fixed.

[antonkomarev]

I think we could separate this method on 2 strict typed methods:

• timestamp(DateTimeInterface $dateTime)
• timestampFromString(string $dateTime)

[atymic]

It's pretty common for laravel to accept mixed types (eg, a date string or a DateTimeInterface).
I generally end up checking if it's a DateTimeInterface and falling back to parsing as a string.

[antonkomarev]

@atymic yeah, but DateTimeInterface doesn't have setTimezone method:
https://www.php.net/manual/en/class.datetimeinterface.php

[Funfare]

RocketChat also accepts DATE_ATOM timestamp, so I changed the code to using that. Then DateTimeInterface works fine

[antonkomarev]

I've replaced ATOM with RFC3339 because its name of the standard (their values are same).