Previously Rails would generate your schema/structure dump based on the
database config name. As noted on the forum in
https://discuss.rubyonrails.org/t/horizontal-sharding-schema-management/78621/6


this is problematic if you want to your shards to share the same schema
dump file.

This is also useful for applications that might have already written a
custom solution for setting the schema dump filename and want to use
something upstream. We allow setting the path for the schema cache, why
not the schema dump too?

To do this I deprecated the old schema_file database task in favor for a
new one that passes the `db_config`. The code to derive the filename
from the db_config has been duplicated there and will be the correct way
to determine filename in the future. Since these values are set on the
config they should be accessible in the config as well.

I did not need to deprecate the behavior of schema_dump because it has
not been included in any release yet (other than the alpha). However the
behavior is the same for false/nil.

Closes #43173
Supercedes #43240

Co-authored-by: Ryan Kerr <leboshi@gmail.com>