refactor(core): route reqsign send via accessor http client

[Xuanwo]

Which issue does this PR close?

Closes #.

Rationale for this change

Services that use reqsign currently wire ReqwestHttpSend with a global default reqwest client. This bypasses accessor-level HTTP client injection and makes reqsign traffic inconsistent with OpenDAL's HTTP abstraction.

What changes are included in this PR?

• Add AccessorInfoHttpSend in opendal-core HTTP utilities.
• Implement reqsign_core::HttpSend for:
  • HttpClient
  • AccessorInfoHttpSend (delegates to AccessorInfo::http_client() at send time)
• Migrate reqsign users to this adapter in:
  • s3, gcs, oss, obs, cos, azblob, azdls, azfile
• Remove reqsign-http-send-reqwest from migrated services.
• Switch S3 endpoint normalization from reqwest::Url to url::Url, and remove the direct reqwest dependency from opendal-service-s3.
• Update core/Cargo.lock accordingly.

Validation:

• cargo fmt --check
• cargo check --all-features
• cargo clippy --all-targets --all-features -- -D warnings

Are there any user-facing changes?

Yes.

Services using reqsign now send signing/credential HTTP requests through the accessor's HTTP client configuration, so injected clients are respected consistently. No public API changes.

AI Usage Statement

Implemented with Codex (GPT-5) in the local repository, with human review before PR submission.

[tisonkun]

Follow up a bit at #7235.

Generally LGTM. Thanks!

[Xuanwo]

Need to address wasm build failure