feat: add non-breaking virtual-hosted-style support

[rhnvrm]

Summary

• add SetUsePathStyle(bool) for explicit path-style vs virtual-hosted-style control
• preserve legacy defaults by surface for non-breaking behavior
• route runtime URLs, presigned URLs, multipart presigned URLs, and direct upload policy URLs through a shared resolver
• add safe virtual-hosted fallback to path-style for dotted HTTPS buckets, localhost/IP endpoints, path-prefixed endpoints, and invalid DNS labels
• include STS token support in CreateUploadPolicies() form fields and policy conditions
• document the addressing behavior and add regression/coverage tests

Why

Issue #36 asks for real virtual-hosted-style support. PR #37 relaxed bucket validation, but it did not fully solve addressing resolution across runtime, presigned, and policy surfaces, and it risked breaking existing callers.

This PR keeps bucket-scoped APIs bucket-scoped, adds explicit client-level addressing control, and preserves existing behavior by default.

Compatibility

• runtime requests keep legacy path-style behavior by default
• presigned URLs keep their legacy defaults by default
• direct CreateUploadPolicies() defaults keep their legacy action URL behavior by default
• callers can explicitly opt into virtual-hosted style with SetUsePathStyle(false)
• explicit virtual-hosted mode safely falls back to path-style when host-style is unsafe/incompatible

Tests

Ran focused regression/feature coverage for the changed addressing surfaces:

go test . -run '^(TestCustomEndpoint|TestGetURL|TestGeneratePresignedURL_LegacyCustomEndpointUsesPathStyle|TestGeneratePresignedUploadPartURL_LegacyCustomEndpointUsesPathStyle|TestCreateUploadPolicies_LegacyDefaultUploadURL|TestSetUsePathStyle_.*|TestGeneratePresignedURL_UsePathStyle.*|TestGeneratePresignedUploadPartURL_UsePathStyle.*|TestCreateUploadPolicies_.*)$'
go test . -run '^(TestS3_GeneratePresignedURL|TestS3_GeneratePresignedURL_Token|TestS3_GeneratePresignedURL_ExtraHeader|TestS3_GeneratePresignedURL_PUT|TestS3_GeneratePresignedURL_ResponseContentDisposition|TestS3_GeneratePresignedURL_URLEncoding|TestS3_GeneratePresignedURL_ObjectKeyEncoding)$'

Closes #36.
Supersedes #37.