Add Windows Named Pipes to Dogstatsd

[Lewis-E]

What does this PR do?

• Adds a NamedPipe BufferReader option to let Dogstatsd use windows specific functionality
• Adopts the multi-instance approach used by winio and the main agent: one server instance per client, spawned in separate tasks.
• Uses DD_DOGSTATSD_WINDOWS_PIPE_NAME for config + DD_DOGSTATSD_PORT is overridden to 0 when pipe name is set
• The issue of multiple functions is NOT solved by this PR. We need to add a uniqueness factor to the envvar to handle multiple functions in the same plan, similar to the AAS extension GUID usage, but this needs to be handled 1) at the compat layer so it can be distributed to the rust mini-agent and the dogstatsd library from a single source of truth, and 2) we can't modify envvars in java, so we will also need to add another communication method to dogstatsd java envvar handling.

Motivation

• When multiple functions run in Windows run on the same plan, only one can connect due to port conflicts for trace agent and dogstatsd agent. The Serverless Compatibility Layer should use named pipes in Windows for transport to prevent this issue.

Additional Notes

• See also: Add Windows Named Pipes to the Trace Agent #58

Describe how to test/QA your changes

• New #[cfg(test)] in integration_tests.rs
• Manual testing in Azure

Separate ticket to add this to self-monitoring when the multiple functions functionality is added: https://datadoghq.atlassian.net/browse/SVLS-8248

[duncanpharvey]

Approved pending resolution of two comments:

• statsd_windows_pipe_name in /info endpoint
• clippy warning (&format!)