[Feat]: Use rustls-tls instead of native-tls for reqwest

[Vensence]

Is your feature request related to a problem? Please describe.

No response

Describe the solution you'd like

The workspace Cargo.toml declares reqwest without default-features = false:

reqwest = { version = "0.12", features = ["json", "stream"] }

This pulls in native-tls → openssl-sys as a transitive dependency. This breaks cross-compilation to musl targets (e.g. x86_64-unknown-linux-musl) and any environment without OpenSSL development headers
installed.

Suggested fix

reqwest = { version = "0.12", default-features = false, features = ["json", "stream", "rustls-tls"] }

Or alternatively, expose TLS backend selection via feature flags:

[features]
default = ["rustls-tls"]
native-tls = ["reqwest/native-tls"]
rustls-tls = ["reqwest/rustls-tls"]

Reproduction

Any downstream crate that depends on agntcy-a2a-client will transitively pull in openssl-sys, even if the downstream crate explicitly uses default-features = false on its own reqwest dependency. Cargo
feature unification merges the features at the binary level, so there is no way for downstream consumers to opt out of native-tls.

cargo tree -i native-tls

native-tls v0.2.18
├── hyper-tls v0.6.0
│   └── reqwest v0.12.28
│       └── agntcy-a2a-client

### Describe alternatives you've considered

_No response_

### Additional context

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### Code of Conduct

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