fix: load the tags manifest asynchronously (#64563)
## What?
A small update to FileSystemCache to replace sync calls with async.
## Why?
`loadTagsManifest` may be called multiple times per request. Since
`loadTagsManifest` is synchronous it blocks the main thread whilst
reading from the file system which could impact server performance.
Replacing these sync calls with async has no impact for consumers of the
FileSystemCache.
Co-authored-by: JJ Kasper <jj@jjsweb.site>
