Make InMemory object store track last modified time for each entry

[gruuya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The InMemory object stores currently spoofs the last_modified field of the fetched ObjectMetas. In some cases, it is important for this value to be correct.

One such case is deducing the DeltaTable version based on a user-provided timestamp in time travel querying—if the last_modified field is wrong the wrong version of the table will be loaded. Admittedly, no one should be using an in-memory store for actual work, but it would still be very handy to have this working for testing.

Describe the solution you'd like
Keep a separate map of last_modified timestamps, and update it on each write call (put, copy, etc.).

Describe alternatives you've considered
Don't use InMem store whenever last_modified field is crucial for the logic.

Additional context
This could be a good first issue.

[tustvold]

Changing storage to be

storage: Arc<RwLock<BTreeMap<Path, (Bytes, DateTime<Utc>)>>>

Makes a lot of sense to me 👍

[gruuya]

Changing storage to be

storage: Arc<RwLock<BTreeMap<Path, (Bytes, DateTime<Utc>)>>>

Makes a lot of sense to me 👍

Agreed, that is indeed more concise!

[tustvold]

label_issue.py automatically added labels {'object-store'} from #3796