Added parameters if_exists and if_not_exists for index operations.

Added parameters if_exists and if_not_exists for index operations.

Fixes: #151
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### Description
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As mentioned in #151, IF EXISTS/IF NOT EXISTS syntax was implemented in SQLAlchemy 2.0. This request adds an ability to use them for index operations.

If the issue implies to implement all the possible cases with these directives, I could continue working on it 🙂

### Checklist
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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.
- [x] 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!**

Closes: #1260
Pull-request: https://github.com/sqlalchemy/alembic/pull/1260
Pull-request-sha: 5ed62d17c349dc175521993c5a62b842f6ac624d

Change-Id: Ic0fec21e20d4a868e9e29275e540f3e09918d1ff