dym: use fn_addr_eq for function pointer comparison

[ovadiagal]

Small fix to rust dynamic modules SDK.

Came across this issue when trying to bump dynamic-modules-examples to 1.38.0:

error: function pointer comparisons do not produce meaningful results since their addresses are not guaranteed to be unique
  --> src/lib.rs:66:1
   |
66 | / declare_all_init_functions!(init,
67 | |     http: new_http_filter_config_fn,
68 | |     network: new_network_filter_config_fn,
69 | |     udp_listener: new_udp_listener_filter_config_fn
70 | | );
   | |_^
   |
   = note: the address of the same function can vary between different codegen units
   = note: furthermore, different functions could have the same address after being merged together
   = note: for more information visit <https://doc.rust-lang.org/nightly/core/ptr/fn.fn_addr_eq.html>
   = note: this error originates in the macro `envoy_proxy_dynamic_modules_rust_sdk::set_factory_once` which comes from the expansion of the macro `declare_all_init_functions` (in Nightly builds, run with -Z macro-backtrace for more info)

Commit Message: dym: use fn_addr_eq for function pointer comparison
Additional Description:
Risk Level: Low
Testing: N/A
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

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Caused by: #44654 was opened by ovadiagal.

see: more, trace.