Add S3 region configuration support and update related validation

[grishy]

close #47

Type of change

• Bug fix
• New feature
• Docs update
• Build/CI

[github-actions]

Merging this branch will decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/grishy/any-sync-bundle/cmd	0.00% (ø)
github.com/grishy/any-sync-bundle/config	37.86% (-5.74%)	👎
github.com/grishy/any-sync-bundle/integration	0.00% (ø)

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/grishy/any-sync-bundle/cmd/root.go	0.00% (ø)	56 (+4)	0	56 (+4)
github.com/grishy/any-sync-bundle/cmd/start.go	0.00% (ø)	1019 (+26)	0	1019 (+26)
github.com/grishy/any-sync-bundle/config/bundle.go	37.57% (-7.24%)	181 (-2)	68 (-14)	113 (+12)	👎
github.com/grishy/any-sync-bundle/config/convert.go	48.15% (-6.52%)	81 (+6)	39 (-2)	42 (+8)	👎
github.com/grishy/any-sync-bundle/integration/bundle.go	0.00% (ø)	0	0	0
github.com/grishy/any-sync-bundle/integration/containers.go	0.00% (ø)	0	0	0

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/grishy/any-sync-bundle/config/bundle_test.go
• github.com/grishy/any-sync-bundle/config/convert_test.go
• github.com/grishy/any-sync-bundle/integration/integration_test.go