Plugin discovery

[c24t]

As discussed in today's SIG meeting, we need to settle on conventions for naming and discovering plugins, i.e. exporters and integrations.

Right now we're using the opentelemetry.ext namespace, and each plugin is a namespace package. One option is to use importlib to discover matching namespace packages.

We may be able to write generic tests to be run by plugin developers. For example, an exporter test that loads each registered exporter and exports one or more spans. Exporter plugins could run this test from the latest version of the core library. Note that this would mean packaging core library tests such that others can run them, and distinguishing exporters from plugins.

This is motivated by #272, the first exporter to graduate from ext/ and get its own separate repo.

[codeboten]

This will be addressed by #760