Allow setting the query string programatically

[lizozom]

This is a follow up on #44377

At the moment, developers may use the following APIs:

const { filterManager, timefilter } = data.query;
filterManager.setFilters(...);
timefilter.timefilter.setTime(...);

To gain programmatic access to the modify SearchBar settings.

However, the value of the query string is inaccessible and is held by each application only.
As a result, there is no way to programmatically change it (and there were frequent requests to enable this behavior).

There should be an additional API to get \ set \ clear the query string.

const { queryString } = data.query;
queryString.get(...);
queryString.set(...);
queryString.clear(...);

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[LeeDr]

We're past 7.6.0 Feature Freeze so if this isn't a bug it should probably bump to v7.7.0.

[lizozom]

Query string can now be modified programatically by using the data.query.queryString.setQuery() API on the data plugin's public contract.