Add message when running composer commands that The Update Frame is being used

[tedbow]

Add message when running composer commands that The Update Framework is being used and it will affect performance.
I think we just need 1 message per operation and just list the repos that are being protected.

This is important because people who are using TUF in UI like Drupal AutoUpdates might not be aware it runs on the Composer level and will therefore affect Composer operations on the command line also. They might just think Composer is slightly slower for no obvious reason

[ergonlogic]

I opened a PR (#125) to resolve this. It's pretty basic.

FYI, $io->info(...) and $io->notice(...) don't seem to print anything at the default verbosity. So I borrowed the technique used in composer-patches (ie. $io->write('<info>...</info>'))

I also moved the "This may impact performance" message from the individual repos, to the message that the plugin is active, since I think we only need to mention that once.

[ergonlogic]

This seems to have failed some tests. I didn't update the tests, since they run under -vvv, and so should still print the info messages, afaict.

[ergonlogic]

The test was only checking for output in STDERR. Now we also check STDOUT, when relevant.