Better memoize instance level variables when creating pipelines

What does this MR do and why?

Without this change each call to secret_instance_variables would:

• trigger a Redis call to check that the cache is still valid
• transform the variables from active record objects into variables items, possibly having some decryption overhead.

The plan
Refactor instance secret variables	!79435 (merged)	this one
Refactor project secret variables	!79850 (merged)
Refactor group secret variables	!79936 (merged)
Bundle them under a single method	!79935 (merged)

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