Skip initializers in the doctor:secrets task for encrypted data

What does this MR do and why?

If encrypted data is being used in initializers, simply initializing the objects to verify the encrypted attributes will fail without a "good" way to figure out which rows are affected.

This adds a way to skip initializers when needed, and does it for the Integration models.

Related to gitlab-org/charts/gitlab#3259 (closed)

How to set up and validate locally

# Create an integration

Integrations::Slack.create(project: Project.last, properties: {})

# Invalidate the secret

Integration.last.update(encrypted_properties: 'Z' * 12)

Run the rake task (bin/rake gitlab:doctor:secrets) and see it doesn't error out & finds the error:

I, [2022-05-03T21:05:25.151824 #994968]  INFO -- : - Integrations::Slack failures: 1

Notice that running it in the console fails, i.e.

[9] pry(main)> Gitlab::Doctor::Secrets.new(Logger.new(STDOUT)).send(:check_model_attributes, {a: 2})
StandardError: can only be used in a Rake environment
from /home/catalin/work/gitlab/gdk/gitlab-development-kit/gitlab/lib/gitlab/doctor/secrets.rb:55:in `check_model_attributes'

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