oembed_providers:oembed_providers CacheTag is not invalidated

Problem/Motivation

When ProviderRepositoryDecorator->getAll() generates its provider list, it caches the list for a week. The cache is not regenerated when either 1) a custom provider is added, updated, or deleted or 2) the Providers URL is updated.

Proposed resolution

Re custom providers, these are config entities. It may be possible simply to override the ConfigEntityBase::getCacheTagsToInvalidate() method to add 'oembed_providers:oembed_providers' as a cache tag for these config entities. Ended up going with the CacheBackendInterface::delete() route instead.

Re global settings, such as the oEmbed Providers URL (and other forthcoming settings), we may need to use CacheBackendInterface::delete() to invalidate 'oembed_providers:oembed_providers' when these settings are updated.

Remaining tasks

Submit patch
Update tests

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

None. This module is still alpha.