Add socket activation support for TCP and UDS servers

[AndreeaDrehuta]

Adds an opt-in "socket-activation" feature that lets tonic servers accept pre-opened listeners passed by systemd for both TCP and Unix domain sockets. Includes examples, systemd unit files and unit tests.

Fixes #2733

Motivation

Under systemd, a service can be started on-demand when a client connects. The manager creates and listens on the socket itself, then passes the already-listening file descriptor to the spawned process via the LISTEN_FDS / LISTEN_PID environment variables.
Consider a gRPC server that receives requests only occasionally. Keeping it running around the clock just to answer the rare request wastes memory and other resources. A better fit for that situation is to let the process run only while it is actually serving requests: the service manager (systemd) holds the socket, starts the server on an incoming connection, and the server can stop itself once it goes idle, after which the manager re-activates it on the next connection.

Solution

Introduces a new, opt-in socket-activation cargo feature (Unix only, disabled by default, so there is no impact on existing users).

When the feature is enabled, socket binding checks for an inherited listener before opening a new one:

TCP: TcpIncoming::bind(addr) adopts an inherited listening socket whose address matches addr, if none was passed in, it falls back to binding normally.
UDS: the Unix domain socket path binding adopts a matching inherited listener when present, otherwise binds as usual.

Implementation details:

A helper (socket_activation.rs) reads LISTEN_PID / LISTEN_FDS, verifies the fds were intended for this process, and returns the first inherited fd that is a listening stream socket matching the requested address.
Behavior is fully backward compatible: with the feature off, or when no fds are passed in, binding works exactly as before.

Also included to make the feature usable and testable:

Example TCP and UDS client/server binaries under examples/src/socket_activation/.
Ready-to-use systemd .socket / .service unit files.
Shell test scripts (test_tcp.sh, test_uds.sh) that exercise the activation flow end to end.
Unit tests covering fd scanning and listening-socket detection.

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