fix: accept any callable of the correct shape as a format reader

[ramsey]

Description

This is a slight change I decided to make while I was updating the README and noticed this oversight.

This PR allows the message loader to accept the following values for the custom format reader:

• Fully-qualified class name
• An already-instantiated instance object of FormatPHP\Format\ReaderInterface
• A callable with the shape callable(mixed[]): FormatPHP\MessageCollection
• The path to a script that returns a callable with this shape

Product requirements and context

How has this been tested?

PR Checklist

• I have added tests to cover my changes.

[qlty-cloud-legacy]

Code Climate has analyzed commit e4e61c9 and detected 1 issue on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	1

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 96.9% (0.0% change).

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