ProviderBucket config entities should have custom provider dependencies

Problem/Motivation

A ProviderBucket config entity may declare its use of one or more custom oEmbed providers (e.g. OembedProvider config entities). This creates a dependency that is not enforced.

Proposed resolution

Add a ::calculateDependencies() method to \Drupal\oembed_providers\Entity\ProviderBucket and add any dependent OembedProvider config entities as dependencies.

Also, add a post_update function to resave all ProviderBucket, thus adding the missing dependencies to existing sites.

Remaining tasks

• Write code
• Update tests

User interface changes

None

API changes

None

Data model changes

None