[GDBMS_cypher] e(), date() and localtime() functions

[GhostVaibhav]

This PR adds the following functions:

• e()
• localtime_c()
• date()

[GhostVaibhav]

The function localtime() is renamed as localtime_c() due to naming conflicts with the default functions present in Postgres.

[emotionbug]

It's a good idea to write tests together so that you can communicate to the other person that the code is working the way you want it to and that this is what you intended.

[GhostVaibhav]

Alright, that means to bundle certain testcases at the top of the respective functions as comments?

[emotionbug]

Nope. refer here.
https://github.com/apache/age/tree/GDBMS_cypher/src/test/regress
postgres/postgres@a04bb65#diff-03ef06b3944648003e81bced9c46d8b3ee47d636239f0f84c558807d693bb57f

[GhostVaibhav]

Added tests for the e() function [b383661] and the date() function [ea9364f]. Need assistance for writing tests for localtime_c().