Add ability to have an optional name when creating a backup

[tbarn]

POST /organizations/{organization}/databases/{database}/branches/{branch}/backups endpoint to create a backup supports a body param for name that would be useful for Terraform providers to be able to use from planetscale-go.

API reference for endpoint: https://api-docs.planetscale.com/reference/create-a-backup

Handy link to backups.go code: https://github.com/planetscale/planetscale-go/blob/ecff34e5422e2eb20435b60429b42012f03f4219/planetscale/backups.go#L77

Might consider adding the option to pass in retention_unit and retention_value too.