fix:basic auth user -> username attribute change

[leggetter]

Fixes:

hookdeck_source_verification.my_authenticated_source: Creating...
╷
│ Error: Value Conversion Error
│ 
│   with hookdeck_source_verification.my_authenticated_source,
│   on main.tf line 26, in resource "hookdeck_source_verification" "my_authenticated_source":
│   26: resource "hookdeck_source_verification" "my_authenticated_source" {
│ 
│ An unexpected error was encountered trying to convert tftypes.Value into sourceverification.basicAuthSourceVerification. This is always an error in the provider. Please report the following to the provider developer:
│ 
│ mismatch between struct and object: Struct defines fields not found in object: username. Object defines fields not found in struct: name.

[leggetter]

@alexluong tested locally:

Terraform will perform the following actions:

  # hookdeck_source_verification.my_authenticated_source will be created
  + resource "hookdeck_source_verification" "my_authenticated_source" {
      + source_id    = "src_vzc72yltwkkrw2"
      + verification = {
          + basic_auth = {
              + password = (sensitive value)
              + username = "example-username"
            }
        }
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

hookdeck_source_verification.my_authenticated_source: Creating...
hookdeck_source_verification.my_authenticated_source: Creation complete after 0s

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.