supabase / auth / 11008788490

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feat: config reloading (#1771)

## What kind of change does this PR introduce?

File based configuration reloading using fsnotify. 

## What is the current behavior?

Currently the Auth config is loaded once from the environment or file
(-c flag) and persists until the service is restarted.

## What is the new behavior?

A new optional flag (long: `--watch-dir`, short: `-w`) has been added.
When present any files with a ".env" suffix will be loaded into the
environment before the `*GlobalConfiguration` is created, otherwise
existing behavior is preserved.

In addition when the watch-dir flag is present a goroutine will be
started in serve_cmd.go and begin blocking on a call to
`(*Reloader).Watch` with a callback function that accepts a
`*conf.GlobalConfiguration object`. Each time this function is called we
create a new `*api.API` object and store it within our `AtomicHandler`,
previously given as the root handler to the `*http.Server`.

The Reloader uses some simple heuristics to deal with a few edge cases,
an overview:

- At most 1 configuration reload may happen per 10 seconds with a +-1s
margin of error.
- After a file within `--watch-dir` has changed the 10 second grace
period begins. After that it will reload the config.
- Config reloads first sort each file by name then processes them in
sequence.
- Directories within `--watch-dir` are ignored during config reloading.
- Implementation quirk: directory changes can trigger a config reload,
as I don't stat fsnotify events. This and similar superfulous reloads
could be easily fixed by storing a snapshot of `os.Environ()` after
successful reloads to compare with the latest via `slices.Equal()`
before reloading.
  - Files that do not end with a `.env` suffix are ignored.
- It handles the removal or renaming of the `-watch-dir` during runtime,
but an error message will be printed every 10 seconds as long as it's
missing.
- The config file passed with -c is onl... (continued)

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