sdkconfig: improve TCP performance

The default TCP window size is too low for anything but low-latency
links. This causes stuttering audio for WIS TTS where the latency
between Willow and WIS is 50ms, even for very short responses. Increase
the default TCP window size to 16 * MSS (1440), and adapt the receive
mailbox size according to the LwIP documentation.

Similar issues could happen with WIS for STT, so let's increase the
default TCP send buffer size also.

Both the TCP send buffer and receive window size were 5740 before this
change.

While at it, also enable TCP SACK, which can improve overal performance
by reducing unnecessary retransmissions.

sdkconfig.defaults

@@ -91,6 +91,10 @@ CONFIG_LWIP_DHCP_GET_NTP_SRV=y
 CONFIG_LWIP_DHCP_MAX_NTP_SERVERS=1
 CONFIG_LWIP_LOCAL_HOSTNAME="willow"
 CONFIG_LWIP_TCPIP_TASK_AFFINITY_CPU0=y
+CONFIG_LWIP_TCP_SND_BUF_DEFAULT=23040
+CONFIG_LWIP_TCP_WND_DEFAULT=23040
+CONFIG_LWIP_TCP_RECVMBOX_SIZE=18
+CONFIG_LWIP_TCP_SACK_OUT=y
 CONFIG_MBEDTLS_CERTIFICATE_BUNDLE_MAX_CERTS=1000
 CONFIG_MBEDTLS_EXTERNAL_MEM_ALLOC=y
 CONFIG_MBEDTLS_HARDWARE_AES=y