Render if_not_exists option for CreateIndexOp and DropIndexOp

[lachaib]

Fixes: #151

Description

Without opinion on whether or not the option if_not_exists or if_exists should be set by default (or configurable via AutogenContext), I think a follow-up on
#151 should be that if_not_exists or if_exists attributes should be that they are rendered during autogen steps.

How I intend to use it: specify the attribute during rewrites of the pipeline

@writer.rewrites(ops.CreateIndexOp)
def create_index_if_not_exist(_autogen_context, _revision, op: ops.CreateIndexOp):
    op.if_not_exists = True
    return op


@writer.rewrites(ops.DropIndexOp)
def drop_index_if_exist(_autogen_context, _revision, op: ops.DropIndexOp):
    op.if_exists = True
    return op

Checklist

This pull request is:

• A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• A short code fix
  • please include the issue number, and create an issue if none exists, which
    must include a complete example of the issue. one line code fixes without an
    issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• A new feature implementation
  • please include the issue number, and create an issue if none exists, which must
    include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!