Cache CI variables used for rules evaluation

What does this MR do and why?

I've imported https://gitlab.com/gitlab-org/gitlab/ locally and used request profiling to figure out some slow points during pipeline creation.

It looks like we're building the variables collection hundreds of times when creating the pipelines(This might explain #343225 (comment 706397676)):

1. Cache predefined project variables:

2. Ci::Pipeline#feeze_period? seems to take of the time from predefined pipeline variables, so we cache it and also cache some of the variables.

3. Contextable#secret_group_variables seems to take most of the time now, but it's already using some sort of memory caching:

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