Add as_any to Conn trait for accessing underlying Conn

[jaredwolff]

This allows you to access the inner connection state of the DTLS connection. Having the connection identity is helpful in server code.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 11.11111% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 61.65%. Comparing base (a97d49e) to head (ee9b09d).

Files	Patch %	Lines
ice/src/agent/agent_test.rs	0.00%	2 Missing :warning:
ice/src/agent/agent_transport.rs	0.00%	2 Missing :warning:
ice/src/agent/agent_vnet_test.rs	0.00%	2 Missing :warning:
ice/src/udp_mux/udp_mux_conn.rs	0.00%	2 Missing :warning:
.../association_internal/association_internal_test.rs	0.00%	2 Missing :warning:
util/src/conn/conn_bridge.rs	0.00%	2 Missing :warning:
util/src/conn/conn_disconnected_packet.rs	0.00%	2 Missing :warning:
util/src/conn/conn_pipe.rs	0.00%	2 Missing :warning:
util/src/conn/conn_udp.rs	0.00%	2 Missing :warning:
util/src/conn/conn_udp_listener.rs	0.00%	2 Missing :warning:
... and 2 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #465 +/- ## ========================================== - Coverage 61.71% 61.65% -0.06% ========================================== Files 529 529 Lines 48828 48852 +24 Branches 12250 12260 +10 ========================================== - Hits 30134 30121 -13 - Misses 9532 9572 +40 + Partials 9162 9159 -3 ```

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[jaredwolff]

Missed this. Thanks @rainliu! I was having a bunch of trouble understanding/getting the downcast to work. Let me try it and then update this PR if it works out.