nanocoap_sock: ensure response address is the same as request address

[benpicco]

Contribution description

If a node has multiple addresses we must reply to a request with the same address on which the request was received.

Since nanoCoAP is not always used as a server and often with only a single address, keep it optional and only enable it when the sock_aux_local module is used.

Testing procedure

CoAP server needs to have two public addresses for this issue to occur:

e.g. set CFLAGS += -DCONFIG_GNRC_NETIF_IPV6_ADDRS_NUMOF=3 in examples/suit_update and do

> ifconfig 5 add 2001:9e8:1416:8f00::1234
success: added 2001:9e8:1416:8f00::1234/64 to interface 5

> ifconfig
Iface  5  HWaddr: 7A:37:FC:7D:1A:AF 
          L2-PDU:1500  MTU:1492  HL:255  RTR  
          Source address length: 6
          Link type: wired
          inet6 addr: fe80::7837:fcff:fe7d:1aaf  scope: link  VAL
          inet6 addr: 2001:9e8:1416:8f00:7837:fcff:fe7d:1aaf  scope: global  VAL
          inet6 addr: 2001:9e8:1416:8f00::1234  scope: global  VAL
          inet6 group: ff02::2
          inet6 group: ff02::1
          inet6 group: ff02::1:ff7d:1aaf
          inet6 group: ff02::1:ff00:1234

CoAP requests to either public address should now be answered with the same public address to which the request was made:

> ncget coap://[2001:9e8:1416:8f00::1234]/.well-known/core -
</.well-known/core>,</riot/board>,</suit/>

> ncget coap://[2001:9e8:1416:8f00:7837:fcff:fe7d:1aaf]/.well-known/core -
</.well-known/core>,</riot/board>,</suit/>

Issues/PRs references

same as #18026

[maribu]

Code looks good and I trust your testing

[riot-ci]

Murdock results

✔️ PASSED

ad8dd8d nanocoap_sock: ensure response address is the same as request address

Success	Failures	Total	Runtime
6919	0	6919	13m:19s

Artifacts

• Documentation preview

[benpicco]

bors merge

[bors]

Build succeeded:

• Murdock
• static-tests
• tools-build-success