[Expression] Add merge prebuilt function for merging JSON object

[cosmicshuai]

issue link: #4014

merge function can take JSON object as input.
merge(jobject1, jobject2, ...)

for example:

json1 = @"{
                   'FirstName': 'John',
                   'LastName': 'Smith',
                   'Enabled': false,
                    'Roles': [ 'User' ]
                   }"

json2 =@"{
                  'Enabled': true,
                  'Roles': [ 'User', 'Admin' ]
                 }"

string(merge(json(json1), json(json2))) will return

{"FirstName":"John","LastName":"Smith","Enabled":true,"Roles":["User","Admin"]}

[tomlm]

[cleemullins]

Please refactor the method that is 1878 lines long.

[tomlm]

🕐

[chrimc62]

[boydc2014]

@cleemullins I saw Shuai created two other tickets tracking your requested changes, i think this one is good to go, what do you think?

[cleemullins]

Blocked on #4208

[cleemullins]

Grandfathered this in, pending the code refactor to Expressions.