feat(sip_client): add auth_username (3CX Auth-ID) and accept SIP INFO / marker-less RFC 2833 DTMF

[eigger]

Closes #318.

1. auth_username — Digest auth ID separate from the extension

3CX v20 (and other PBXs) issue an Authentication ID that differs from the
extension number. Digest username= always carried the extension, so REGISTER
failed with 403 Credentials don't match.

sip_client:
  username: "100"             # extension: From / To / Contact
  auth_username: "<auth-id>"  # optional: Digest username only
  password: "<password>"

Applied to both the REGISTER and the INVITE challenge. Omitting the option
keeps the current behaviour exactly (username is used for Digest).

2. DTMF receive: marker-less RFC 2833 + SIP INFO

on_dtmf only fired on telephone-event packets that carried the RTP marker
bit, so Yealink DECT handsets (W70B/W73H) and the 3CX Android app never
triggered a door relay.

• The telephone-event decoder now reports a digit once per (event, RTP
  timestamp) and ignores the marker bit entirely. Since every packet of one
  event repeats its timestamp, this also swallows the retransmitted end
  packets — firing on the end bit instead would have fired ~3x per keypress.
  State is reset on session start/stop, so an identical packet in the next call
  is not mistaken for a duplicate.
• SIP INFO with application/dtmf-relay (Signal=1) or application/dtmf
  now fires the trigger, answered with 200 OK as before. The body is parsed
  through Signal=, so Duration=250 is not misread as digit D; * / #
  are accepted literally and as event codes 10 / 11. INFO outside an
  established call is answered but does not fire the trigger.

In-band audio tones remain undecoded.

Existing on_dtmf automations are unchanged.

Tests

Digit decoding, the receive de-duplication and the INFO body parser live in
dtmf.h / dtmf.cpp with no ESPHome dependency, and are covered by 10 new
cases in tests/native/sip_sdp/test_dtmf.cpp (wired into run.sh and the
suite README).

• tests/native/sip_sdp/ — all pass
• esphome compile tests/components/sip_client/test.esp32-idf.yaml — succeeds
  (the test YAML now also sets auth_username, so CI covers the new option)

On-device verification against 3CX v20, a Yealink desk phone, Yealink DECT and
the 3CX Android app is still pending — @AlexW85 offered to test in #318.

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