nanocoap_sock: add nanocoap_sock_url_connect()

[benpicco]

Contribution description

This introduces a sock_urlpath() function that just returns a pointer to the path component of a URL without having to copy it.

This moves the URL parsing code out of nanocoap_get_blockwise_url() into nanocoap_sock_url_connect() and save some stack by not having to allocate a CONFIG_SOCK_URLPATH_MAXLEN sized buffer there.

Testing procedure

The url command in tests/nanocoap_cli still works

main(): This is RIOT! (Version: 2022.07-devel-91-g135d3-sock_urlpath)
nanocoap test app
All up, running the shell now
> url get coap://[fe80::90a7:a6ff:fe4b:2e32%5]/.well-known/core
url get coap://[fe80::90a7:a6ff:fe4b:2e32%5]/.well-known/core
offset 000: </>;ct=40;rt="tag:chrysn@fsfe.or
offset 032: g,2022:fileserver"

Issues/PRs references

useful for #17962, #17937

[maribu]

ACK. Code changes are unscary and mostly reorganising already tested code. Since there are unit tests that Murdock will execute (at least on native) for the newly exposed features, IMO no manual testing is required.