zenoh plugins statically linked in a zenoh app

[JEnoch]

Currently (v0.5.0-beta.8) the zenoh plugins are dynamic libraries that are loaded by zenohd at startup.
They just must implement 2 #[no_mangle] functions:

// return the list of command lines arguments expected by the plugin
fn get_expected_args<'a, 'b>() -> Vec<clap::Arg<'a, 'b>>;

// start the plugin with command line arguments
fn start(runtime: Runtime, args: clap::ArgMatches<'static>);

Now we would like to be able to use the zenoh plugins in any zenoh Rust application, but in a static way (i.e. just declaring the plugins in the application's cargo.toml and calling them explicitly in the main() operation).

Use case:
The zenoh-dds-plugin can be built as a standalone zenoh application named zenoh-bridge-dds.
We would like to administrate it at runtime in the same way as zenohd: via an admin space accessible through the REST API. The REST API is implemented as a zenoh plugin. But we don't want the zenoh-bridge-dds to have the ability to dynamically load any plugin. The solution is to statically link zenoh-bridge-dds with the zenoh-rest plugin.
And as the code of the DDS/zenoh bridge itself should eventually be also a zenoh plugin (to be loadable by zenohd), the zenoh-bridge-dds will finally just by a main program starting the 2 plugins that are linked in static.

[JEnoch]

The current design using 2 #[no_mangle] function cannot work with statically linked plugins: the declaration of those functions by different plugins override each other, leading only 1 plugin to be accessible by the main().

Conditional compilation of #[no_mangle] attribute depending on the crate type (cdylib or rlib) doesn't work either, as long as rust-lang/rust#20267 is not fixed.

The most appropriate solution seems to follow the design suggested in https://michael-f-bryan.github.io/rust-ffi-guide/dynamic_loading.html:

• define a trait to be implemented by each plugin struct
• define a macro to be used by each plugin implementation to generate a "standardized" #[no_mangle] function to instantiate the plugin struct
• the plugin manager in zenohd will call the #[no_mangle] function for each plugin
• a standalone application which statically links with the plugins will directly instantiate and drive each plugin struct

[JEnoch]

Implemented by #114.
Applied in eclipse-zenoh/zenoh-plugin-dds#38.