Support adding prefix/subfolder under an S3 bucket for Redshift

**Is your feature request related to a problem? Please describe.**
Ccube does not support adding prefix/subfolder under an S3 bucket in .env file. If you try to hard-code path after the bucket name it will have errors like:
InvalidBucketName: Bucket name shouldn't contain '/', received (your wrong path here)

**Describe the solution you'd like**
Maybe to have CUBEJS_DB_EXPORT_BUCKET changed to CUBEJS_DB_EXPORT_PATH

**Describe alternatives you've considered**
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