Allow using system certificate store for TLS

[msiemens]

Allows reqwest to use the system certificate store when validating a TLS certificate e.g. for OIDC.

Fixes #203

[brocaar]

Thanks @msiemens, you are right. However, I think the line rustls-tls (which is an alias for rustls-tls-webpki-roots) should be replaced by rustls-tls-native-roots?

Please see:

• https://docs.rs/reqwest/latest/reqwest/#optional-features and
• https://docs.rs/crate/reqwest/latest/features#rustls-tls-native-roots (which enables rustls)

[brocaar]

Added to the above, I think it better to just rely on the OS CA certificates instead if of bundling CA certificates with ChirpStack. What do you think?

[msiemens]

I've adapted the PR to not use the bundled certificates but only the OS certificates.

I think it better to just rely on the OS CA certificates instead if of bundling CA certificates with ChirpStack. What do you think?

I agree, especially considering that Mozilla's CA certificate collection for instance is updated multiple times a year and using the OS certificates would allow updating the certificates without having to update a running ChirpStack installation.