feat(elements): add ComponentNgElementStrategy to public API

[remackgeek]

Adds ComponentNgElementStrategy, ComponentNgElementStrategyFactory to the public API,
allowing third-party libraries to extend the default strategy behavior
(api docs to come in a separate PR)

Fixes #26112, #33490

PR Checklist

Please check if your PR fulfills the following requirements:

• [x ] The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

Issue Number: #26112, #33490

What is the new behavior?

ComponentNgElementStrategy, ComponentNgElementStrategyFactory are now public

Does this PR introduce a breaking change?

• Yes
• [x ] No

Other information

[IgorMinar]

Hi there. Thank you for your contribution to Angular.

I'm sorry but at the moment we are not interested in allowing the default strategy to be part of our public API and to be extended by third parties because it would tie our hands in improving and modifying this default in the future.

Please feel free to implement the NgElementStrategy strategy api and feel free to use the ComponentNgElementStrategy as an inspiration for your implementation.

[remackgeek]

It was suggested yesterday by @gkalpak, in #24181, based on the publicApi annotations in the code.

If you don't want to expose the strategy, just exposing the ComponentNgElementStrategyFactory (and changing the create() signature to return NgElementStrategy) would let you change the strategy at will, but still let library developers wrap existing behavior.
Right now to do this, I have to create a dummy element with the default strategy, just so I can reach into it and get the strategy to wrap it.

[IgorMinar]

thanks for the additional context @remackgeek. I just spoke with @gkalpak and suggested some alternative solutions. He's going to post a comment with the details. I think the problem you are trying to fix is worth fixing, it's just that the change you proposed here is not heading in the right direction. @gkalpak will share more details. thanks!

[gkalpak]

So, basically, making the strategy (and corresponding factory) part of the public API will increase the maintenance cost by making it harder for us to make changes to them in the future (either fixinf bugs or making improvements).

If anyone wants to define their own strategy, they need to implement the NgElementStrategy interface. This makes things a little more complicated/verbose for people wanting to make a small change to the ComponentNgElementStrategy by entending it, but we feel the benefits are worth it.

Regarding the problem with Zones (#24181) in particular, we decided it is worth fixing in @angular/elements, i.e. with something similar to #24185. So, let's continue the discussion on #24185.

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