Make it easier to get source field values from media items

Problem/Motivation

A media item always references a source, and we frequently want to access field values from that source in MediaSource plugins. Instead of repeating similar code in many places, it will be easier to write, easier to read, and more reliable to provide a method for this common task.

The code for this was already introduced in #2831944: Implement media source plugin for remote video via oEmbed. We are splitting off this issue in the hope of making the new method available a little sooner.

Proposed resolution

Add the method getSourceFieldValue() to MediaSourceInterface and implement it in MediaSourceBase.

Completed tasks

1. Add the method to the interface.
2. Add the method to the base class.
3. Add a test.

Remaining tasks

1. Write a change record.

User interface changes

None

API changes

Add a new method to MediaSourceInterface.

Most classes that extend MediaSourceBase will not have to make any changes.

1. Any class that implements the interface without extending the base class will have to implement this method.
2. If the source value is not the main property of the source field, as returned by mainPropertyName(), then the MediaSource plugin will need to override this method.

Without this method, we use code like this in MediaSource plugins:

    $source_field = $this->getSourceFieldDefinition($media->bundle->entity)->getName();
    /** @var \Drupal\Core\Field\FieldItemInterface $source_field_item */
    $source_field_item = $media->get($source_field)->first();

    $main_property = $source_field_item->mainPropertyName();
    $resource_url = $source_field_item->get($main_property)->getString();

After adding this method, that can be simplified:

    $resource_url = $this->getSourceFieldValue($media);

Data model changes

None