[Feature] make header row optional for dbt seeds

[graciegoheen]

Describe the feature

If I've configured my seed's column names in a yml file, I don't want to also keep a header row up-to-date in my csv seed file.

seeds:
  - name: my_seed
    columns:
      - name: country_code
        data_type: varchar(2)
      - name: country_name
        data_type: varchar(32)

country_code,country_name
US,United States
CA,Canada
GB,United Kingdom

Instead, I should be able to configure my seed to not need a header row if I've configured my column names in yml already.

seeds:
  - name: my_seed
    config:
      header: false
    columns:
      - name: country_code
        data_type: varchar(2)
      - name: country_name
        data_type: varchar(32)

US,United States
CA,Canada
GB,United Kingdom