Add filling in last seen value to zabbix datasource fill mode

[yurtesen]

Is your feature request related to a problem? Please describe.

#1109 describes the problem with graphs when items are used with discard preprocessing option. The #1082 helped to progress the issue half way. Now we can have null values in place of missing values.

However null does not solve the specific problem. There should be an option to provide previous value in place of missing values. This is implemented in other data sources when such filling is used: grafana/grafana#12756

Describe the solution you'd like
I would like to have the option of having previous value in place of null when the new data frame format is used.

Describe alternatives you've considered
There is no alternative.

Additional context
With null values the graph looks like this:

If null value is graphed as connected then it looks incorrectly like this:

If previous was available then the graph should have looked correctly like this:

Because the missing value is the same as the previous available value.

[github-actions]

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