datetime and decimal should be logical types

[damccorm]

Currently datetime and decimal are implemented as primitive types in the Java SDK. They should be logical types as documented in https://s.apache.org/beam-schemas

Imported from Jira BEAM-7554. Original Jira may contain additional context.
Reported by: bhulette.

[Abacn]

.take-issue

[Abacn]

This issue was raised again in May 22, per this comment in Jira

ilango-mediaagility:
Due to this issue, cross language transforms are affected. For example, when using ReadFromJdbc transform in python, we get the decimal and datetime as logical types, but they are encoded differently than the underlying representation types, resulting in incorrect values.

This is blocked by #21433