[databroker] optional dependency for TLS

[AkhilTThomas]

The current version of databroker always adds tonic::tls as a mandatory dependency. Can this be moved a feature based dependency ? since tls is anyways an optional feature

[lukasmittag]

could be optional I guess. But then we need to change it in the Dockerfile too, There we need it because otherwise tls won't work with the containers. and also we need to update the documentation. So could e an optional dependency, but I think it would be simpler to keep it mandatory. But I'm open for arguments if there are some to make it optional.

[AkhilTThomas]

The reason was to get a leaner version of databroker for systems that don't support openssl. adding tls makes it pull lot more dependencies like rusttls, hyper-tls. this can be avoided if tls is not used.

[SebastianSchildt]

I think it might make sense, we should however for "default" builds and things we release for Linux always default to include TLS, as this is the right thing™ to do. So in terms of Cargo would be good, if we could express it as antifeature "no-tls" that needs to be enabled explicitly, if that is possible.

[SebastianSchildt]

maybe this can be some inspiration https://github.com/seanmonstar/reqwest/pull/375

[AkhilTThomas]

Yes, something like Cargo.toml

default = ["tls"]  
notls = [ tonic/transport , tonic/channel, tonic/prost]
tls   = [ tonic/transport , tonic/channel, tonic/prost, tonic/tls ]

and then wrap everything with tls in #[cfg(feature = "tls")]

[AkhilTThomas]

I have created a PR #638. @SebastianSchildt Can you review ?