remove config & defaults globals

[tobert]

In the first release of otel-cli, configuration was a bunch of globals in the cmd package. This later was moved into the Config struct, replacing global vars with global config.Field. Now that Config is settled down and in good shape, it's time to clean up the config and defaults globals to instead pass around data.

This will require a few changes to function signatures to pass Config around. Some unit tests may change too, and some cleaner unit testing options will become available when it's done.

[tobert]

Made some progress on this in #198

[tobert]

config global is gone. only otlpclient.Diag remains, will be gone soon

[tobert]

For the most part, the config global is gone. Diag remains and will be iterated out over time, but it's so low risk and high utility it might not matter.