feat: export custom model (like as dbt models .sql)

[AntoineGiraud]

what

Hi, this is a PR to add the --schema_name when we do a custom export

example :
datacontract export tests/fixtures/custom/export/datacontract.yaml --format custom --template tests/fixtures/custom/export_model/template.sql --schema-name line_items

why

i wasn't able to do it previously and it felt odd to have a for loop to export a custom dbt model ... that is supposed to be 1 model per file

to be discused

I tried to update the README.md acordingly

This PR is in draft to open the discusion with materials to chat on

i'm also puzzled on if the "data_contract" passed to the template.file will be an ODCS exclusively
or could it still be the ol' datacontract spec ?!

though for now the method is
def to_custom(data_contract: OpenDataContractStandard, template_path: Path) -> str:

PushToProd checks

• Tests pass
• ruff format
• README.md updated (if relevant)
•  CHANGELOG.md entry added