ORC-1390: [C++] Support schema evolution from string group to decimal…

…/timestamp

### What changes were proposed in this pull request?
1. Support schema evolution from `{stirng, char, varchar}` to `{decimal,timestamp,timestamp_instant}`
2. Fix a bug that cannot convert from `varchar` to `varchar` and `char` to `char`

### Why are the changes needed?
To support Schema evolution at c++ side.

### How was this patch tested?
UT passed

### Was this patch authored or co-authored using generative AI tooling?
NO

Closes #1949 from ffacs/ORC-1390.

Authored-by: ffacs <[email protected]>
Signed-off-by: Gang Wu <[email protected]>