feat(p-multiselect): add template to customize the dropdown

[sandrotonon]

See discussion #2157

Usage

If provided, the dropdown template replaces the whole input component. Clicking the templates content triggers the visibility of the multiselect-overlaypanel. It also maintains full keyboard-control.

Example Code

<p-multiselect
  [options]="countries"
  [(ngModel)]="selectedCountries"
  placeholder="Select Countries"
  optionLabel="name">
    <ng-template let-value pTemplate="dropdown">
        <p-button icon="pi pi-flag-fill" severity="secondary"></p-button>
    </ng-template>
</p-multiselect>

Outcome

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1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
primeng	⬜️ Ignored (Inspect)	Visit Preview		Jun 17, 2024 1:37pm

[mertsincan]

Thanks a lot for your contribution! Could you please update your PR and create an issue? Thank you!

[sandrotonon]

Since the PR was not able to merge automatically, I decided to close it. Maybe I will reopen a new PR, if I manage to find some time.