feat: endpoint and region config for s3 storage

[killme2008]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Main changes:

• Adds #[serde(default)] attribute to all config options.
• Adds region and endpoint config for s3 storage.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #820 (f70c0fc) into develop (041cd42) will decrease coverage by 0.10%.
The diff coverage is 40.47%.

@@             Coverage Diff             @@
##           develop     #820      +/-   ##
===========================================
- Coverage    85.27%   85.17%   -0.11%     
===========================================
  Files          418      418              
  Lines        55224    55258      +34     
===========================================
- Hits         47094    47065      -29     
- Misses        8130     8193      +63     

Flag	Coverage Δ
rust	85.17% <40.47%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/cmd/src/standalone.rs	56.30% <ø> (+6.75%)	⬆️
src/datanode/src/datanode.rs	47.72% <ø> (ø)
src/datanode/src/instance.rs	29.89% <0.00%> (-13.68%)	⬇️
src/frontend/src/frontend.rs	38.88% <ø> (ø)
src/meta-srv/src/metasrv.rs	70.40% <ø> (ø)
tests-integration/src/test_util.rs	86.89% <0.00%> (-12.62%)	⬇️
src/cmd/src/toml_loader.rs	100.00% <100.00%> (ø)
src/object-store/src/test_util.rs	0.00% <0.00%> (-100.00%)	⬇️
... and 9 more

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[fengys1996]

LGTM