new feature: Add "last access time" to `Metadata`

### Feature Description

`Metadata` should have an optional `last_access_time` field.

### Problem and Solution

When implementing a bounded file cache, it's useful to be able to prioritize garbage collection based on access time. I believe most filesystems support this metadata, but regardless we can provide it as an `Option`.

I would add support for filesystems first. I'm not sure how many object stores could support it.

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [x] Yes, I am willing to contribute to the development of this feature.

Start by locating the Metadata type and the filesystem metadata implementations. Trace how metadata is collected and exposed, then identify the relevant filesystem tests or adapter coverage. Done means Metadata can optionally carry last_access_time for filesystem backends, with unsupported sources remaining representable as None.

Written by the indexing model from the issue text.