Create a 'bc' top level item in serialization.settings config object

This is follow to the API First Initiative in DrupalCon Vienna

Problem/Motivation

Currently in core/modules/serialization/config/schema/serialization.schema.yml there are 2 elements that start with "bc_".
These are backwards compatibility settings to determine if certain serializers should be used.

There may more(many more?) settings needed like these as we address problems in core's serialization process.

Proposed resolution

To better organize this we should create a top level "bc" time in the 'serialization.settings' config object. All bc settings would be put under this. So
serialization.settings.bc_primitives_as_strings would be changed to serialization.settings.bc.primitives_as_strings

Remaining tasks

1. Determine if \Drupal\serialization\RegisterSerializationClassesCompilerPass::normalizerBcSettingIsEnabled would need to be updated. Or would it just work.
2. Determine BC policy for this change 😜.Presumablyy we would have to still have read the existing keys until 9.0.
3. Write patch

User interface changes

None

API changes

None

Data model changes

Schema changes to 'serialization.settings' config object