gcs: compatible with old gcs bug (#677)

[ti-srebot]

cherry-pick #677 to release-5.0-rc
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr 689

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/br.git pr/689:ti-srebot:release-5.0-rc-3de76b2ec3f2

---

What problem does this PR solve?

solve #675

What is changed and how it works?

for case 1: add a check before Read & Write meta file, decide which prefix to use.
for case 2: check whether sst file is in Prefix(after #647) or in PrefixSlash(before #647), adjust proper prefix for TiKV request.

Check List

Tests

• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Note

• Fix the issue that br 4.0.9 cannot restore br 4.0.8 gcs backup files.

[overvenus]

LGTM

[overvenus]

Please fix build.

[kennytm]

lgtm