fix: Ensure that API key and secret are not shown in the Terrafom plan output

[diglesias-te]

This fixes #12

I've been testing this module and I've realized that the values for aqua_api_key and aqua_api_secret are being shown in the Terraform plan output when the single/lambda module is used.

This is not an issue on the single/trigger module as these variables are marked as sensitive there and thus not shown.

Before:

# module.aqua_aws_onboarding.module.single[0].module.lambda.aws_lambda_invocation.generate_volscan_external_id_function will be created
  + resource "aws_lambda_invocation" "generate_volscan_external_id_function" {
      + function_name   = (known after apply)
      + id              = (known after apply)
      + input           = jsonencode(
            {
              + ApiUrl            = "..."
              + AquaApiKey        = <plaintext_api_key>
              + AquaSecretKey     = <plaintext_api_secret>
              + AutoConnectApiUrl = "..."
            }
        )
      + lifecycle_scope = "CREATE_ONLY"
      + qualifier       = "$LATEST"
      + result          = (known after apply)
      + terraform_key   = "tf"
      + triggers        = (known after apply)
    }

After:

  # module.aqua_aws_onboarding.module.single[0].module.lambda.aws_lambda_invocation.generate_volscan_external_id_function will be created
  + resource "aws_lambda_invocation" "generate_volscan_external_id_function" {
      + function_name   = (known after apply)
      + id              = (known after apply)
      + input           = (sensitive value)
      + lifecycle_scope = "CREATE_ONLY"
      + qualifier       = "$LATEST"
      + result          = (known after apply)
      + terraform_key   = "tf"
      + triggers        = (known after apply)
    }

We use automated workflows to deploy Terraform code on GitHub, as such, we need to ensure that these values remain hidden. Let me know what you think, thanks.

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