Add TimeZone property to AtataContext

[YevgeniyShunevych]

The need of this feature was raised in #483.

Basically, with this feature it becomes possible to set UTC or another time zone to keep all Atata date/time values (like log event timestamp) within the set time zone.

Implementation

Add to AtataContext:

public TimeZoneInfo TimeZone { get; }

Gets the time zone. The default value is TimeZoneInfo.Local.

TimeZone can be set as usually thru new AtataContextBuilder methods:

public AtataContextBuilder UseTimeZone(TimeZoneInfo timeZone);

public AtataContextBuilder UseTimeZone(string timeZoneId);

public AtataContextBuilder UseUtcTimeZone();