add support for specifying secondary indexes with to_sql
Support for creating secondary indexes in `to_sql` via the parameter `indexes`.

The code also tries to avoid creating duplicate indexes when `keys` is specified in `SQLTable`.
(Once the PR for passing the `keys` this would be useful)

This PR introduces a new method in `SQLTable` called `_is_column_indexed` so that subclasses can easily override or change the logic.

Interesting future work will be to also support creating compound indexes.

does this have an open issue for this? (IIRC there is)
 I didn't find this exact issue.

https://github.com/pydata/pandas/issues/7984 and https://github.com/pydata/pandas/issues/9084 are similar in that they also deal with to_sql / sql indexes but not specifically specifying which columns should be indexes.

I am interested in maybe picking up https://github.com/pydata/pandas/issues/7984 as well after this PR. Together with a PR for specifying multi-column / compound indexes.
 hmm @trbs this PR looks like it should close those 2 issues, or is there something that is indicated in those that is not in your PR?
 #7984 is about unique indexes... this PR would only add an index to the column not unique. I add that to this PR.

#9084 is basically about adding more parameters to get_schema(...) to that it includes the index=.... and index_label=... parameters that `to_sql` has. (so its about the dataframe index not database indices)

I could do the patch for #9084 as well since it looks kinda trivial. But it's probably better to do that in a separate PR ?
 @jorisvandenbossche 
 status? @jorisvandenbossche 
 can you rebase / update?
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/12904?src=pr) is 85.24% (diff: 100%)

> Merging [#12904](https://codecov.io/gh/pydata/pandas/pull/12904?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #12904   diff @@
==========================================
  Files           140        140          
  Lines         50563      50575    +12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43103      43114    +11   
- Misses         7460       7461     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [54ab5be...a092198](https://codecov.io/gh/pydata/pandas/compare/54ab5be78fac125ae6efcd650d8d3f418e60754d...a0921987d5fc69b2b1b80c7b24988f1d8d3ff530?src=pr)
 @jorisvandenbossche status? btw happy to rebase again if Pandas wants to merge this PR. closing as stale