Command alias implementation

[vicky1999]

Motivation

• Command alias implemented for sub commands
• If a command need to be aliased, we need to add the alias and the command to aliases map in main.cc.
• The key should be combined with a # character

Related to #8914 #8875

Screenshots:

Description

• I have created an alias with store ping to store info. After adding the alias in aliases map, the application was able to recognize both the commands and giving the same result

[vicky1999]

@edolstra @fricklerhandwerk
I have implemented the command level aliasing as per the requirement. Kindly review the changes.

[Ericson2314]

We found a simple-stupid solution for nix store ping -> nix store info in #8975 (comment). Let's use that to finish off that PR, and then return to this?