Add use of Mozilla's root certificates for TLS client communication as a feature

[harlem88]

astarte-device-sdk-rust/src/transport/mqtt/config/tls.rs

Line 135 in e1cd0b8

rustls_native_certs::load_native_certs().map_err(PairingError::Native)?;

Currently for a TLS communication we use the root_certificate provided by the platform. In some scenarios the platform could not provide a valid root certicate, so we can provide it via webpki-roots.

Example:

#[cfg(feature = "webpki")]
let root_cert_store = RootCertStore {
    roots: webpki_roots::TLS_SERVER_ROOTS.into(),
};