feat: add decimal validation for numeric precision and scale supported by Spanner

[vi3k6i5]

feat: add decimal validation for numeric precision and scale supported by spanner
Fixes #339 🦕

[olavloite]

This seems good to my untrained Python eyes, with a small question on whether it is logical to call the validate function for all values, or whether it would be better to move the if-check for the type out of the validation function.
(You should in any case await approval from other reviewers before merging)

[vi3k6i5]

This seems good to my untrained Python eyes, with a small question on whether it is logical to call the validate function for all values, or whether it would be better to move the if-check for the type out of the validation function.
(You should in any case await approval from other reviewers before merging)

moved the method after a check. and also moved it to base client as @larkee suggested.

[larkee]

LGTM 👍 You could remove test_w_numeric if you wanted since test_w_numeric_precision_and_scale_valid should have more coverage.