Add support for basic query EXPLAIN

[ajnavarro]

Explain query output has a really difficult to implement format: https://dev.mysql.com/doc/refman/8.0/en/explain-output.html

To be able to show some information to the user, we can do a small workaround and implement a specific, MySQL language compatible EXPLAIN statement format:

[EXPLAIN|DESCRIBE|DESC] FORMAT=TREE [query]

The output will be one column, one string row with the string tree representation of the generated logical plan:

mysql> EXPLAIN FORMAT=TREE SELECT * FROM repositories where repository_id != "blah";
+------------------------------------------------------------+
|                            plan                            |
+------------------------------------------------------------+
|Project(repositories.repository_id)                         |
| └─ PushdownProjectionAndFiltersTable                       |
|     ├─ Columns(repositories.repository_id)                 |
|     ├─ Filters(NOT(repositories.repository_id = "blah"))   |
|     └─ Table(repositories)                                 |
|         └─ Column(repository_id, TEXT, nullable=false)     |
+------------------------------------------------------------+

Maybe is worth it add more plans like parsed, analyzed and optimized.