feat: allow to get the network address server is listening on

[haoqixu]

Add an Addr() method to fetch the network address Server is listening on. I think this may be useful for the supervisor to start a server with 0 port to allocate an ephemeral port and get the allocated port for rendering an agent config.

[codecov[bot]]

Codecov Report

Attention: 129 lines in your changes are missing coverage. Please review.

Comparison is base (efddaa2) 76.11% compared to head (6d08f1f) 72.50%. Report is 31 commits behind head on main.

Files	Patch %	Lines
internal/certs.go	0.00%	118 Missing :warning:
client/internal/httpsender.go	88.00%	2 Missing and 1 partial :warning:
client/internal/mockserver.go	85.71%	2 Missing and 1 partial :warning:
server/httpconnection.go	0.00%	2 Missing :warning:
client/internal/clientcommon.go	90.90%	1 Missing :warning:
client/internal/packagessyncer.go	0.00%	1 Missing :warning:
server/serverimpl.go	95.65%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #217 +/- ## ========================================== - Coverage 76.11% 72.50% -3.62% ========================================== Files 24 25 +1 Lines 1834 2022 +188 ========================================== + Hits 1396 1466 +70 - Misses 326 446 +120 + Partials 112 110 -2 ```

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[tigrannajaryan]

Thank you @haoqixu !