agent: allow auto-auth to use an existing token

[jasonodonnell]

This PR changes auto-auth to accept a token during the creation of the auth handler. This feature will be used with persistent caching where a token is decrypted from the cache file and passed into auto-auth. Reusing an existing token is critical to maintaining leases previously owned by the token loaded from the cache.

Currently this feature only attempts to load the token on the first run and falls back to auto-auth if that's unsuccessful. Retry logic could be added if valuable, but likely if there's an issue loading retrying won't help.

Once the token is no longer renewable, the renewer will fall back to auto-auth and reauthenticate.

To test this feature you can run the unit tests:

$ cd ./command/agent
$ go test -run 'TestTokenPreload*'

[tvoran]

The tests and example bootstrap script work for me. Just a couple questions.

[calvn]

One small note, otherwise looks good!

[tomhjp]

LGTM, just one suggestion to simplify control logic around backoffs.