Add name to Spree::Address

[filippoliverani]

solidusio#3458

[filippoliverani]

I created a migration to add name with firstname, space and lastname concatenation.
I addeed a rake task to backfill and index name column to leave more control over data migration, it could be time consuming on large db.
filippoliverani@c2bb23b

[filippoliverani]

To favor backward compatibility during deprecation period, writes on firstname and lastname update name too. Writes on name keeps firstname and lastname updated splitting on the first space to have a fallback during deprecation period.
Presence validation on firstname and name has been updated accordingly.

I updated Address to considered in lookup, merge and equality only non deprecated attributes. I added a Spree::Name value object to handle name parsing during deprecation period. I replaced all deprecated attributes occurrences in core with name.
I added the preference flagshow_deprecated_attributes to leave the possibility of viewing deprecated attributes in API responses (and potentially anywhere else) and ease the way to the upgrade.
filippoliverani@1554586

[filippoliverani]

I introduced deprecation warnings on firstname, lastname and full_name and updated all specs to be more specific when checking expectaions on Spree::Deprecations mocks to avoid overlapping warnings.

[filippoliverani]

I replaced all deprecated attributes occurrences in api with name. APIs responses show firstname and lastname only if show_deprecated_attributes is set to true.

[filippoliverani]

I replaced all deprecated attributes occurrences in frontend and backend with name. Deprecated fields are rendered in HTML views and APIs responses views instead of name field only when show_deprecated_attributes preference is set to true.

[filippoliverani]

I updated api docs to OpenAPI Specification 3 to be able to mark properties as deprecated in responses and inputs schemas.