add preflight check (without app status check and move primary check)

[MiaAltieri]

Issue

There is no pre-flight check for upgrades

Solution

Copying over old-preflight check to new upgrade implementation

Summary + Future PRs

My goal is to simplify PR review and separate out:

1. already reviewed code for pre-flight checks
2. new code that is necessary for pre-flight checks

In order to separate out the new pre-flight checks with the old ones. I will make two PRs:

1. This one with the old code. This code was already reviewed in the previous implementation + adding back int tests
2. A second one with more checks such as:

• moving the MongoDB primary
• checking all units of juju application are active
• some unit tests

[Mehdi-Bendriss]

Thank you Mia!

[carlcsaposs-canonical]

apologizes for the delay; planning to review after implementing pre-upgrade-check structure in opensearch (https://warthogs.atlassian.net/browse/DPE-4128)

@MiaAltieri please let me know if you'd like me to review earlier

[carlcsaposs-canonical]

here's the pre-upgrade check shared code canonical/opensearch-operator#262 (runs pre-upgrade-check on action and automatically after juju refresh before first unit upgrade)