add a option to provide customized bucket endpoint

[davies]

Closes #1003

[codecov-commenter]

Codecov Report

Merging #1008 (c121edf) into main (630263d) will increase coverage by 2.59%.
The diff coverage is 18.18%.

@@             Coverage Diff              @@
##               main    #1008      +/-   ##
============================================
+ Coverage     37.20%   39.79%   +2.59%     
- Complexity       33      149     +116     
============================================
  Files           146      146              
  Lines         23560    23563       +3     
  Branches        422      422              
============================================
+ Hits           8766     9378     +612     
+ Misses        13617    12891     -726     
- Partials       1177     1294     +117     

Impacted Files	Coverage Δ
cmd/gateway.go	0.00% <0.00%> (ø)
cmd/mount.go	0.00% <0.00%> (ø)
.../src/main/java/io/juicefs/JuiceFileSystemImpl.java	57.19% <66.66%> (+57.19%)	⬆️
...java/src/main/java/io/juicefs/JuiceFileSystem.java	79.59% <0.00%> (-2.23%)	⬇️
pkg/meta/sql.go	66.51% <0.00%> (-0.56%)	⬇️
pkg/meta/redis_metrics.go
pkg/meta/metrics.go	0.00% <0.00%> (ø)
pkg/meta/redis.go	69.32% <0.00%> (+0.16%)	⬆️
pkg/vfs/handle.go	77.31% <0.00%> (+0.84%)	⬆️
pkg/vfs/writer.go	75.93% <0.00%> (+1.01%)	⬆️
... and 5 more

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