[UDFs] support default values for arguments

[graciegoheen]

Housekeeping

• I am a maintainer of dbt-core

Short description

allow users to set a default_value for their function arguments, setting a default_value makes that argument optional

Acceptance criteria

• allow users to set a default_value for their function arguments (for warehouses that support this)
• In any warehouse that supports defaults, a defaulted argument would become optional

    arguments: # Optional
      - name: string # Required
        data_type: string # Required
        description: string # Optional
        default_value: # Optional, having a default value would mean that the argument is optional