output logging messages to stderr so --dry-run output can be piped

[briceburg]

to "behave" as expected IMO we should print login messages and other logging type information to stderr so that output can be redirected to configuration files;

e.g. running

aws-sso-util configure populate  --dry-run > foo.config

should a produce a file that starts with [profile ...] instead of

Logging in to https://ACME.awsapps.com/start/
Gathering accounts and roles
Dry run for 146 profiles
[profile ACME-...]
...

[benkehoe]

I changed the logging specifically to use stderr only for errors with #54. I don't think --dry-run with piped stdout is the right way to do this. You can currently accomplish this with:

AWS_CONFIG_FILE=foo.config aws-sso-util configure populate

but I could add an explicit option for --config-file to make it less obscure.