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:

planetscale-go/planetscale/backups.go

Line 77 in ecff34e

path := backupsAPIPath(createReq.Organization, createReq.Database, createReq.Branch)

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