feat: Add tls-native, tls-rustls features to allow building w/o Rustls

[jirutka]

No description provided.

[simonsan]

Hey there, we are currently working on a relatively big refactor to an own rustic_backend crate, see #73.

We need to focus first on that one, before we are being able to merge this.

[simonsan]

Also regarding the topic, as there has been no issue, which needs to be fixed, and we haven't discussed it, could you elaborate a bit on the background:

• What's your use case?
• Why is this change needed?
• What does it improve/solve?

[aawsome]

Agree to @simonsan - when the backend refactor is done, the TLS things will be all in the crate rustic_backends, so we better wait for the refactoring #73 to land!

Besides this, if there are use cases where you need other TLS options, I would very much agree to adding the possibilites as features. It might be that the depending crate(s) (e.g. opendal) need to support those features, too...

[jirutka]

What's your use case?

Packaging rustic for Alpine Linux.

Why is this change needed and what does it improve/solve?

Bundling (static linking) TLS library is a very bad idea from a security perspective, and Linux distributions try to avoid it. Also, rustls/ring doesn’t support all platforms supported by Rust, e.g. ppc64le and s390x.

[simonsan]

@jirutka if the features are mutually exclusive, could you throw a compile error in lib.rs, in case both are turned on?

#[cfg(all(feature = "tls-native", feature = "tls-rustls "))]
compile_error!("feature \"tls-native\" and feature \"tls-rustls \" cannot be enabled at the same time. Please disable one of them.");

[jirutka]

if the features are mutually exclusive, could you throw a compile error in lib.rs, in case both are turned on?

Sure, done.

[aawsome]

Now that #73 is merged, this PR can continue...
@jirutka: Your changes should now go to crates/backend. Can you move this or do you need support?

Also, now that we have added opendal, this should be also treated - however, I don't know if this is already supported by opendal; this may need a bit of research.