Summary:
This patch adds support to write and read a user-defined timestamp size record in log writer and log reader. It will be used by WAL logs to persist the user-defined timestamp format for subsequent WriteBatch records. Reading and writing UDT sizes for WAL logs are not included in this patch. It will be in a follow up.
The syntax for the record is: at write time, one such record is added when log writer encountered any non-zero UDT size it hasn't recorded so far. At read time, all such records read up to a point are accumulated and applicable to all subsequent WriteBatch records.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11433
Test Plan:
```
make clean && make -j32 all
./log_test --gtest_filter="*WithTimestampSize*"
```
Reviewed By: ltamasi
Differential Revision: D45678708
Pulled By: jowlyzhang
fbshipit-source-id: b770c8f45bb7b9383b14aac9f22af781304fb41d
