Schema usage for Oracle databases

As requested in #265, this PR adds the ability to use schemas in Oracle. As you may know, they differ a little bit from the ones in Postgres since they're tightly coupled with users, but it's possible to grant access to another users.

This PR reuses the --schema from Postgres.

Say, a user1 has granted access to user2 to their schema, and the latter wants to see the user1's tables, it only has to add the schema flag to the CLI, as done for Postgres databases.

# Oracle
$ dblab --host localhost --user user2 --db FREEPDB1 --pass password --port 1521 --driver oracle --limit 50 --schema user1
$ dblab --url 'oracle://user2:password@localhost:1521/FREEPDB1' --schema user1

In the image the user2 can see the user1's tables (TEST_TABLLE in this case).

Fixes #265

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

QA the change by creating two different users, granting access to the second one, so that, the second could see the first user's tables.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings