Add 'enter-admin-container' to easily enable and enter admin container

[tjkirch]

Description of changes:

d54e5ec enable-admin-container: use 'apiclient set'
17d4a7a enable-admin-container: only set if not already
    Changing the setting restarts services, which is unnecessary if it's
    already enabled, and restarting services disconnects existing users of
    the admin container.
f5de0a2 Add 'enter-admin-container' to easily enable and enter admin container

This is based on the new apiclient exec from bottlerocket-os/bottlerocket#1802, which allows people to enter the admin container without SSH; this PR makes that easier to use from the control container.

Testing done:

Tested the enable-admin-container changes first:

• If it's not already enabled, it does so.
• If it's already enabled, it doesn't! No more disconnections!

Then enter-admin-container:

• If it's not already enabled, it uses enable-admin-container correctly, then enters.
• If it's already enabled, it enters quickly.

Also confirmed that it dies and messages as expected if it takes too long for the admin container to start, along with a bunch of improbable error cases found before the code was clean.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.