Summary:
This diff introduces the changes to add a YSQL method `yb_get_current_hybrid_time_lsn` which returns the current time value in the form of `HybridTime`.

The usage of the method will produce an output similar to the following:

```
yugabyte=# select yb_get_current_hybrid_time_lsn();
 yb_get_current_hybrid_time_lsn
--------------------------------
            7109163364038488064
(1 row)
```
Jira: DB-14611

Test Plan: Run existing tests.

Reviewers: skumar, siddharth.shah, sumukh.phalgaonkar, stiwary, utkarsh.munjal, devansh.saxena

Reviewed By: devansh.saxena

Subscribers: yql, ybase, ycdcxcluster

Differential Revision: https://phorge.dev.yugabyte.com/D40822