[API] Improve List Functions Tag Filtering [1.6.x]

[quaark]

https://iguazio.atlassian.net/browse/ML-6640

Previously to filter and attach the tags to the list functions method, we:

1. Got all tags with the given tag name, join on their functions and return function uids
2. Query all functions that match the resulting uids
3. For each function:
   a. Query function's tags and add all the permutations of (function, tag) to the resulting list.

This is basically runs O(n) queries where n is the amount of functions.

Now we:

1. Query all functions, joined* on the function tags, filtered by tag name. (This will already return the permutations)
2. For each function build the function object from the resulting function and tag permutation.

*outer join if no specific tag was requested so we return functions that are tag-less, and a simpler join when querying on a specific tag as in that case tag-less functions are irrelevant.

This now is just 1 query to the DB.