Implement Socket::getpeername() API

[SeppoTakalo]

Description

This is equivalent of POSIX getpeername() function. It allows to
get remote address associated with the socket.
For example:

socket *s = server.accept();
SocketAddress remote;
s->getpeername(&remote);

This is a new API so should go out on 5.11, and not in 5.10 patch releases.

NOTE: This PR replaces the change proposed in #8565

Pull request type

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

[SeppoTakalo]

@kjbracey-arm and @ARMmbed/mbed-os-ipcore Please review.

[unsignedint]

Sweet

[0xc0170]

/morph build

[mbed-ci]

Build : FAILURE

Build number : 3590
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8651/

[cmonr]

Does this function need a return value?

[SeppoTakalo]

Yes, thank you.

[cmonr]

Error log: http://mbed-os.s3-eu-west-1.amazonaws.com/builds/8651/FAIL/NUMAKER_PFM_M2351/ARMC6/b8c59cbbc86d3151dd81ab0311033a10e35f00fc_build_log_NUMAKER_PFM_M2351_ARMC6.txt

[SeppoTakalo]

Build error fixed and added a unit test for InternetSocket::getpeername()

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3610
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8651/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3214
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8651/

[mbed-ci]

Test : FAILURE

Build number : 3386
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8651/3386

[cmonr]

Note: This PR is now a part of a rollup PR (#8733).

In order to expidite remaining CI testing, this PR has been bundled into the above rollup PR.

No further work is needed here, as once that PR is merged, this PR will also be closed and marked as merged.

If any more commits are made in this PR, this PR will remain open and have to go through CI on its own.