refactor(effects): Refactor types in effects, expose metadata key

[alex-okrushko]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

PR is based off #2190

What kind of change does this PR introduce?

• Cleanup types within effects (remove any)
• expose [CREATE_EFFECT_METADATA_KEY] key on the createEffect and clean up the tests.

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for c13aab5 at https://previews.ngrx.io/pr2194-c13aab5/

[timdeschryver]

Seems like the commits are squashed in 1b70ff1 @alex-okrushko, was this the intention?
Asking because I'm not sure how we want to merge #2191, #2194, and #2195