fix(resource_te_bgp): add default for use_public_bgp

[raul-te]

All public bgp_monitors get removed immediately after applying:

  # thousandeyes_bgp.bgp_test will be updated in-place
  ~ resource "thousandeyes_bgp" "bgp_test" {
        id                       = "2930988"
      - use_public_bgp           = true -> null
        # (12 unchanged attributes hidden)

      - bgp_monitors {
          - monitor_id = 6784 -> null
        }
      - bgp_monitors {
          - monitor_id = 9216 -> null
        }

This contribution fixes that problem.