Supporting setting a default DIALECT for search commands

[shacharPash]

Closes #146
Now the user can set default dialect when creating the Search commands instance, or with SetDefaultDialect(int? defaultDialect).

Examples of setting DIALECT 2 as a default dialect:

ConnectionMultiplexer redis = ConnectionMultiplexer.Connect("localhost");
IDatabase db = redis.GetDatabase();

// Option 1:
var ft = db.FT(2);

// Option 2:
var ft = db.FT();
ft.SetDefaultDialect(2)

[codecov-commenter]

Codecov Report

Patch coverage: 97.18% and project coverage change: +0.32 🎉

Comparison is base (2dcac07) 93.25% compared to head (a1fe79b) 93.58%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   93.25%   93.58%   +0.32%     
==========================================
  Files          78       78              
  Lines        4625     4769     +144     
  Branches      426      448      +22     
==========================================
+ Hits         4313     4463     +150     
+ Misses        189      177      -12     
- Partials      123      129       +6     

Impacted Files	Coverage Δ
src/NRedisStack/Search/SearchCommandsAsync.cs	92.85% <81.25%> (+1.39%)	⬆️
src/NRedisStack/Search/SearchCommands.cs	98.11% <92.00%> (+1.77%)	⬆️
src/NRedisStack/Search/AggregationRequest.cs	99.57% <99.36%> (+1.10%)	⬆️
src/NRedisStack/ModulePrefixes.cs	100.00% <100.00%> (ø)
src/NRedisStack/Search/Query.cs	98.15% <100.00%> (-0.26%)	⬇️
src/NRedisStack/Search/SearchCommandBuilder.cs	89.09% <100.00%> (+4.26%)	⬆️

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