netsocket: add get_remote_peer() method to InternetSocket

[unsignedint]

Description

With the TCP API changes in 5.10 it seems like there is now no easy way to access the address information of the remote peer, because it is not exposed in the protected _remote_peer member. This PR adds a get method to expose it as a const pointer.

The end goal is to replicate previous behaviour from 5.9:

TCPServer srv;
TCPSocket client_sock;
SocketAddress client_addr;
...
srv.accept(&client_sock, &client_addr);
printf("Client connected: %s", client_addr.get_ip_address());

And now we can write:

client_sock = sock.accept();
client_addr = client_sock->getpeername();

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

I have labeled this a "fix" but it can be considered an enhancement, too, I guess.

[0xc0170]

Thank you for the contribution, we will review and provide feedback (someone from the @ARMmbed/mbed-os-ipcore team)

[SeppoTakalo]

I feel like it should be getpeername() as in POSIX standard.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/getpeername.html

All other functions inherit the name and behaviour from POSIX as well.

[unsignedint]

@SeppoTakalo I have renamed to getpeername() and moved up one level to the InternetSocket class, presumably its could also be useful for UDP connections.

[unsignedint]

Also, sorry, I'm still waiting on signed CLA >_<

[kjbracey]

Discussed this with @SeppoTakalo - it's probably best to incorporate the API all the way up in the base Socket interface - it is conceptually required for its base accept.

This is related to #5922 - we concluded there that getsockname was too hard on too many platforms, but getpeername would be a good idea. And accept has since made it necessary.

If it goes all the way up to Socket then TLSSocketWrapper can also implement it by passing through.

[kjbracey]

For lifetime reasons and general POSIX compatibility - aligning with the way we've mapped other functions - this should be prototyped as virtual nsapi_error_t getpeername(SocketAddress *address), assuming implementing a pure virtual in Socket as suggested.

(Other calls already omit the POSIX length parameter for addresses, on the assumption SocketAddress has space for all possible address types...)

[SeppoTakalo]

Park this change.

We as a IP networking team will do this change so that it will become official part of Mbed OS Socket API.

[SeppoTakalo]

API change submitted in #8651

[0xc0170]

API change submitted in #8651

@unsignedint Please review 8651.

We will close this PR as 8651 extends it. Thank you for your first contribution