feat(forms): easier updating of validators on existing controls

[thelgevold]

Please check if the PR fulfills these requirements

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

What kind of change does this PR introduce? (check one with "x")

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

This will add support for setting and clearing validators on exiting controls. It's already possible to do this by manipulating the .validator property on the control, but I am proposing this as an enhancement of the existing API.

The use case is dynamic forms where user actions in the form might require validation rules on any given control to change dynamically. The goal is to make the validator updates easier.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[kara]

Seems like you could use coerceToValidator here?

this.validator = coerceToValidator(newValidator);

[kara]

If there is a way to set the validator, seems like there should be a way to set and clear async validators as well. Is there a reason that wasn't added?

[thelgevold]

@kara That is a good point. I have added async counter parts and addressed your other comments as well,

Thanks!

[kara]

Thanks for the PR! LGTM.

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