Add full outer join support

[ilongin]

Our current API in both DataChain.merge() and internal DatasetQuery.join() support only inner and left outer join by setting inner flag to True or False.

We should add ability to do full outer join, as it's needed in some use cases like when comparing two chains.

Note that SQLite doesn't support full outer join out of the box, in earlier versions which we still support, so we will need to do some workaround with union all and two left joins.

[dmpetrov]

@ilongin is this somehow related to #721?

[ilongin]

@ilongin is this somehow related to #721?

@dmpetrov yes it is