[Spike] Investigate how to allow complex object in filter object in API Explorer

[dhmlau]

Originated from #2208.

Description

swagger-ui, the library powering our API Explorer, unfortunately does not support complex objects in query strings. This spike is to investigate if/how we can allow it.

Extracting from #2208 (comment) from @bajtos

Based on OAI/OpenAPI-Specification#1706 (comment) (cross-posted below), maybe we can switch filter encoding from deepObject style into JSON?

Here is an experiment we can try:

1. Let's keep @param.query.object with no changes for backwards compatibility.
2. Introduce a new helper @param.query.jsonObject that will use JSON encoding instead of deepObject style.
3. Modify the function parseJsonIfNeeded to detect JSON encoding and parse the string value - see packages/rest/src/coercion/coerce-parameter.ts
4. Update our example Todo controller to use @param.query.jsonObject - see examples/todo/src/controllers/todo.controller.ts
5. Start the Todo app, open API Explorer and check how is the filter parameter handled.

Acceptance Criteria

- [ ] Review the workaround posted by various users in #2208

• Try out the experiment from @bajtos (listed above)

[bajtos]

An important aspect to consider: if we switch the spec of format argument from deepObject-style to json-content, how is it going to affect the actual REST API? Will it mean that clients will be no longer able to send queries like find?filter[limit]=1 and will have to always use JSON encoding only (find?filter={"limit":1})? Can we find a way how to preserve support for both flavors?

[deepakrkris]

For having the "include" parameter in filter condition, filter["include"]= does not work whereas passing filter as deep object works.
example:
http://localhost:3000/todos/1?filter=%7B%22include%22%3A%5B%7B%22relation%22%3A%22todoList%22%7D%5D%7D

[deepakrkris]

url encoding of json can be done with javascript built in function encodeURIComponent(stringified-json)

[deepakrkris]

In the open api spec (https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#encoding-object) , explanation for explode setting is,

When this is true, property values of type array or object generate 
separate parameters for each value of the array, or key-value-pair of the map.
For other types of properties this property has no effect.

[deepakrkris]

spike #4141 tries to test passing complex object as uri encoded values in query params

[deepakrkris]

@bajtos the spike #4141 works perfectly fine, API explorer supports url encoding of complex object in query params when implementing steps suggested by you.

Please ignore my previous comment.

cc: @dhmlau