(1/n) Add file watcher (meta-pytorch#1464)

Summary:

Currently Monarch sits in the middle of log forwarding, ie if Monarch crashes a child proc's logs would be lost. 

 Here we add  `LogFileMonitor`, which will be the replacement for `LogFileWriter` and `LogTailer`.

`LogFileMonitor` enables "process spawners" to attach the monitor to specific files (e.g. a custom file where stdout/stderr are teed), which propagates updates to the provided `log_channel`. This will effectively move Monarch out of the critical path for logging, more context on [doc](https://docs.google.com/document/d/1TSrsFqlXRPEhKOyQhTKd1XyY9JrjLH6y_Z5iGnrnzBo/edit?tab=t.0#heading=h.vvg43v4o545o)

- Enables creation of temp log files
- Adds a file watcher that propagates updates on file modifications
- Add missing logging tests

Differential Revision: D84111031

Author: pablorfb-meta

hyperactor_mesh/Cargo.toml

@@ -66,6 +66,7 @@ libc = "0.2.139"
 mockall = "0.13.1"
 ndslice = { version = "0.0.0", path = "../ndslice" }
 nix = { version = "0.30.1", features = ["dir", "event", "hostname", "inotify", "ioctl", "mman", "mount", "net", "poll", "ptrace", "reboot", "resource", "sched", "signal", "term", "time", "user", "zerocopy"] }
+notify = "5"
 opentelemetry = "0.29"
 pin-project = "1.1.10"
 preempt_rwlock = { version = "0.0.0", path = "../preempt_rwlock" }
@@ -87,6 +88,7 @@ bytes = { version = "1.10", features = ["serde"] }
 itertools = "0.14.0"
 maplit = "1.0"
 proptest = "1.5"
+tempfile = "3.22"
 timed_test = { version = "0.0.0", path = "../timed_test" }
 tracing-test = { version = "0.2.3", features = ["no-env-filter"] }