[Python] pyarrow.concat_tables did not really enable the unify_schemas, it only supply unify_schemas options but not actually enable it

### Describe the bug, including details regarding any error messages, version, and platform.

### Describe the enhancement requested

in R there is `concat_tables(..., unify_schemas = TRUE)`(https://arrow.apache.org/docs/r/reference/concat_tables.html)
In python this function does not allow concat table with different schema.

> unify_schemas
>
> If TRUE, the schemas of the tables will be first unified with fields of the same name being merged, then each table will be promoted to the unified schema before being concatenated. Otherwise, all tables should have the same schema.

I tried the promote_options="permissive" which does not seem to do the trick.

```
t1 = pa.Table.from_pydict({'a': [1., 2], 'b': [{'k1': 0, 'k2':1}, {'k3': 'ok'}]})
t2 = pa.Table.from_pydict({'a': [1., None], 'b': [{'k1': None}, {'k3': None}]})

pa.concat_tables([t1, t2], unify_schemas=True, promote_options="permissive")
```

It will have `ArrowTypeError: struct fields don't match or are in the wrong order: Input fields: struct output fields: struct`

I expect this will be just ok

```
cdef cppclass CConcatenateTablesOptions" arrow::ConcatenateTablesOptions":
c_bool unify_schemas
CField.CMergeOptions field_merge_options
```

We can see the bool field is already defined in there, but there is no way to set value in the actual function
https://github.com/apache/arrow/blob/f98a13250d10dba248a2bb85989d6b80265e82d8/python/pyarrow/table.pxi#L5169

in the code you only set the field_merge_options which is option of unify_schema().
I think you either should initialize the unify_schemas to true, oh you need to actually handle the keyword unify_schemas. Otherwise, the field merge options actually not in effect at all.

### Component(s)

Parquet, Python