Support allowed_index_name_length method

This pull request address the error by supporing allowed_index_name_length method.

```ruby
$ ARCONN=oracle ruby -Itest test/cases/migration/index_test.rb -n test_internal_index_with_name_matching_database_limit
Using oracle
Run options: -n test_internal_index_with_name_matching_database_limit --seed 27961

# Running tests:

E

Finished tests in 0.045876s, 21.7979 tests/s, 0.0000 assertions/s.

  1) Error:
test_internal_index_with_name_matching_database_limit(ActiveRecord::Migration::IndexTest):
ArgumentError: Unknown key: internal
    /home/yahonda/git/rails/activesupport/lib/active_support/core_ext/hash/keys.rb:70:in `block in assert_valid_keys'
    /home/yahonda/git/rails/activesupport/lib/active_support/core_ext/hash/keys.rb:69:in `each_key'
    /home/yahonda/git/rails/activesupport/lib/active_support/core_ext/hash/keys.rb:69:in `assert_valid_keys'
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_schema_statements.rb:127:in `add_index'
    test/cases/migration/index_test.rb:79:in `test_internal_index_with_name_matching_database_limit'

1 tests, 0 assertions, 0 failures, 1 errors, 0 skips
```

Oracle enhanced adapter does not need extra length for index operations
this returns the index_name_length .

Refer https://github.com/rails/rails/pull/8613 for detail.