remote_storage: make last_modified and etag mandatory (#7126)

## Problem

These fields were only optional for the convenience of the `local_fs`
test helper -- real remote storage backends provide them. It complicated
any code that actually wanted to use them for anything.

## Summary of changes

- Make these fields non-optional
- For azure/S3 it is an error if the server doesn't provide them
- For local_fs, use random strings as etags and the file's mtime for
last_modified.

libs/remote_storage/tests/test_real_s3.rs

@@ -118,7 +118,7 @@ async fn s3_time_travel_recovery_works(ctx: &mut MaybeEnabledStorage) -> anyhow:
     // A little check to ensure that our clock is not too far off from the S3 clock
     {
         let dl = retry(|| ctx.client.download(&path2, &cancel)).await?;
-        let last_modified = dl.last_modified.unwrap();
+        let last_modified = dl.last_modified;
         let half_wt = WAIT_TIME.mul_f32(0.5);
         let t0_hwt = t0 + half_wt;
         let t1_hwt = t1 - half_wt;