Pad queries when EDNS0 is enabled.

Behavior is compliant with
https://tools.ietf.org/html/draft-ietf-dprive-padding-policy-04

EDNS0 is only enabled when the netcontext requests it, i.e. in DNS-over-TLS
mode.

Bug: 69623036
Bug: 64133961
Test: Wireshark verified. Integration tests echo padding and pass.
Change-Id: I5ef600e02a572d281441e890cc981614f150629b