Do not try to load date format when custom date format is provided but not declared

Problem/Motivation

When you call \Drupal::service('date.formatter')->format($timestamp, $type = '', $format = 'd.d.d.d') Drupal tries to load the date format based on the type because it is not custom but will not found any format for empty string. This can lead to unnecessary calls.
See profiler images before/after in 🐛 Set missing datetime type Active .

Steps to reproduce

Proposed resolution

Teach people via phpdoc non-empty-string (phpstan) and set internal the type to custom when format is provided or medium, when not.

Remaining tasks

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User interface changes

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Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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