Add tmpfs IOFS (#1212)

* Define an interface for IOFS handling. Add no-op and temporary directory implementations.

* Move IOFS stuff out into separate file, add basic tmpfs implementation for linux only

* Switch between directory and tmpfs based on platform and config

* Respect FN_IOFS_OPTS

* Make directory iofs default on all platforms

* At least try to clean up a bit on failure

* Add backout if IOFS creation fails

* Add comment about iofs.Close

api/agent/iofs_notlinux.go

@@ -0,0 +1,22 @@
+// +build !linux
+
+package agent
+
+import (
+	"context"
+	"errors"
+)
+
+type tmpfsIOFS struct {
+	directoryIOFS
+}
+
+func (t *tmpfsIOFS) Close() error {
+	return t.directoryIOFS.Close()
+}
+
+func newTmpfsIOFS(ctx context.Context, cfg *Config) (*tmpfsIOFS, error) {
+	return nil, errors.New("tmpfs IOFS not supported on macOS")
+}
+
+var _ iofs = &tmpfsIOFS{}