fix(typeahead): improve 'ngbTypeahead' typings

[maxokorokov]

Improves "search" function typings by using standard rxjs OperatorFunction type and making search function optional

// before
@Input() ngbTypeahead: (text: Observable<string>) => Observable<readonly any[]>;

// after
@Input() ngbTypeahead: OperatorFunction<string, readonly any[]>| null | undefined;

Fixes #3907

[codecov-io]

Codecov Report

Merging #4038 (c953042) into master (67be4ab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4038   +/-   ##
=======================================
  Coverage   88.07%   88.08%           
=======================================
  Files         112      112           
  Lines        3589     3591    +2     
  Branches      663      664    +1     
=======================================
+ Hits         3161     3163    +2     
  Misses        358      358           
  Partials       70       70           

Flag	Coverage Δ
e2e	46.55% <44.44%> (-0.09%)	⬇️
unit	87.37% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/typeahead/typeahead.ts	96.06% <100.00%> (+0.06%)	⬆️

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