BR 4.0.9 is incompatible with 4.0.8 when using GCS to store backup

[overvenus]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Backup TiDB to GCS with a URL has no tail '/' via BR 4.0.8.
Restore TiDB from GCS.

2. What did you expect to see?

Restore success.

3. What did you see instead?

Restore fails.

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This incompatibility is caused by #647. We need to add a fullback to compatible with older backup (<=4.0.8)

[kennytm]

The bug fixed by #647 has two effects.

1. If the storage is set as gcs://bucket/prefix, the SSTs are written correctly to gcs://bucket/prefix/*.sst but the backupmeta is written wrongly to gcs://bucket/prefixbackupmeta.
2. If the storage is set as gcs://bucket/prefix/, the backupmeta is written correctly to gcs://bucket/prefix/backupmeta, but the SSTs are written wrongly to gcs://bucket/prefix//*.sst (note the extra slash).

Case 1 can be supported simply by adding a --backupmeta-file flag to specify an alternative path of the backupmeta.

Case 2 is hard to fix when #647 is in effect since we Trim() all slashes, meaning -s gcs://bucket/prefix// is equivalent to -s gcs://bucket/prefix, and it won't let TiKV find gcs://bucket/prefix//*.sst.

[kennytm]

Moving (renaming) objects in GCS: https://cloud.google.com/storage/docs/gsutil/commands/mv

Like S3, moving is equivalent to Copy + Delete and is not free.