TemplateBuilder issue with Siddhi objects and Json mapper custom payload mapper

[pierre-borckmans]

Description:
The following app will produce proper json format for the context field:

@App:name("Test siddhi objects json mapping")
define trigger EverySecond at every 1 second;
@sink(type='log', @map(type = 'json'))
define stream LogStream (context object);
from EverySecond select 
  map:create(
    "x",123,
    "y","abcd"
  ) as context      
insert into LogStream;

This will output the following

{ "context": { "x": 123, "y": "abcd" } } 

However, when using a @payload in the mapper as follows, the context object is not formatted as proper json:

@App:name("Test siddhi objects json mapping")
define trigger EverySecond at every 1 second;
@sink(type='log', @map(type = 'json', 
    @payload("""{
  "context": {{context}}   
}
""")
  )
)
define stream LogStream (context object);
from EverySecond
select 
  map:create(
    "x",123,
    "y","abcd"
  ) as context      
insert into LogStream;

will output the following:

{ "context": {x=123, y=abcd}, }

---

I understand this might be complex to fix as the TemplateBuilder is unaware of which mapper (json in this case) is used.

At the end of the day, what I need is a way to produce custom json output from a stream, while being able to use siddhi objects formatted as json.
Note that this is somewhat related to siddhi-io/siddhi-map-json#81 because if I could use the same map as in a source, I would not need a custom @payload