Implement Sort Key per-column direction feature

[miguilimzero]

This pull request implements the Sort Key per-column direction feature using the following syntax:

$table->sortKey([['f_name', 'asc'], ['l_name', 'desc']]);

The developer must use the second argument to set the direction for all columns or set a per-column direction for all columns:

$table->sortKey(['f_name', 'l_name'], 'desc'); // Correct (as before)

$table->sortKey([['f_name', 'asc'], ['l_name', 'desc']]); // Correct (now implemented)

$table->sortKey(['f_name', ['l_name', 'desc']]); // Throw exception

$table->sortKey(['f_name', ['l_name', 'asc']], 'desc'); // Throw exception

Note: A dummy index name was added to the shard key & sort key to support the array column syntax (This prevents Laravel from trying to auto-generate the index name from the column name). It does not impact the schema generation since the driver does not use this name attribute.

@carlsverre Let me know if this was the intended behavior! I didn't understand exactly how it was supposed to be implemented.

[carlsverre]

Do you mind adding one or two tests for the failure cases? Just to make sure that we don't regress in the future. Otherwise this PR looks solid! Thank you!

[miguilimzero]

@carlsverre done!