feat:use parse_decimal for generic_string_to_decimal conversion.

[himadripal]

Which issue does this PR close?

• Closes DataFusion should support casting strings such as "4e7" to decimal datafusion#10315
• Follows up - based on the (Fix: Support for e notation using existing parse_decimal in string to decimal conversion #6905 (comment))
• depends on feat: add rounding logic and scale zero fix fro parse_decimal to match parse_string_to_decimal_native behavior #7179 - tests will pass when this one is merged.

Few important consideration -

• Existing string to decimal conversion uses parse_string_to_decimal_native
• parse_string_to_decimal_native does not have support for e-notation
• parse_string_to_decimal_native does rounding at scale, not truncate
• parse_decimal an existing method has e-notation support and use elsewhere
• add rounding support in parse_decimal
• moved string to decimal conversion to use parse_decimal to get support for e-notation.

This PR is a 3rd one to break up #6905 , this one add rounding logic to parse_decimal to match the behavior in existing parse_string_to_decimal_native.

Closes #.

Rationale for this change

At present, string to decimal conversion does not support e-notation, in arrow, parse_string_to_decimal_native is called to get generic string to decimal. parse_decimal on the other hand is used from generic parse method and it has e-notation support. This PR is adding rounding and scale 0 handling to match the behavior or parse_string_to_decimal_native method. Then we can replace parse_string_to_decimal_native call with parse_decimal. This way, we will get e-notation support too.

What changes are included in this PR?

Are there any user-facing changes?

[himadripal]

#7179 - adds the rounding logic and with that merged, tests will pass. All failed tests are due to parse_decimal truncates instead of round, existing parse_string_to_decimal_native defaults to rounding.

I can make changes to test now and change again when #7179 is merged..