feat: reload profile credentials when the shared credentials file changes

[locle2302]

Problem

A mount started with --profile resolved its credentials once, at mount time, and kept them for the life of the process. When an external rotator rewrote ~/.aws/credentials and deactivated the old key, every read and write on the mount failed with 403 until goofys was restarted.

What changed

Profile mounts now go through sharedFileProvider, a credentials provider that watches the shared credentials file and re-resolves the profile when it changes.

• Change detection compares size and mtime, sampled at most once every 10 seconds, and the fingerprint is captured before the file is read so a rotation landing mid-read is not mistaken for the version that was loaded.
• Resolution runs through session.NewSessionWithOptions with shared config enabled, so role_arn, source_profile, credential_process and SSO profiles keep their full chain instead of being pinned to the raw static keys in ~/.aws/credentials. Expiry of any STS or SSO credentials underneath is honoured as well.
• A failed reload (for example the rotator has the file truncated at that moment) keeps the previously loaded credentials and logs the failure instead of surfacing an I/O error on the mount. An unreadable or removed credentials file likewise keeps the credentials already in use, logged once.
• Credentials are always returned for a profile mount, so a mount can no longer fall through to the process global s3Session that an earlier mount may have built for a different profile.

Mounts that pass explicit keys, that set RoleArn, or that rely on AWS_PROFILE / the implicit default profile are unchanged.

Testing

go test ./api/common/ passes. New coverage: rotation picked up through Get() without an explicit Expire(), a rotation that preserves the file's timestamp, a failed reload keeping the last known good credentials, a profile declared only in ~/.aws/config, and an unknown profile. The tests pin AWS_SHARED_CREDENTIALS_FILE, AWS_CONFIG_FILE, IMDS and ECS credential env vars, and save/restore the s3Session global so they stay hermetic and order independent.