ffi: Add support for serializing a KeyValuePairLogEvent as a nlohmann::json object.

[LinZhihao-723]

Description

Note: this PR should not be merged before #507 and #511
This PR implements a method in KeyValuePairLogEvent to serialize it into a nlohamnn::json object. KeyValuePairLogEvent, as the output of Deserializer in key-value pair IR format, will be processed by the ffi libraries. Each ffi library has to turn this C++ object into some dynamic data structure in their language. Ideally, KeyValuePairLogEvent can be serialized into msgpack byte sequence. However, there are two problems we need to solve:

1. In the current format, arrays are stored as an unstructured JSON string. We need to parse the JSON array to build the msgpack array.
2. It isn't that straightforward to use the official C++ msgpack library to construct an in-memory map and then serialize it into bytes.

Therefore, we decided to serialize KeyValuePairLogEvent into nlohmann::json object instead for now. nlohmann::json provides a better support to construct an in-memory map, and also provides methods to serialize the result into either msgpack byte sequence or JSON string.

With this PR, we should have all the necessary pieces to integrate serialization/deserialization features of the key-value pair IR format into our ffi libraries.

Validation performed

• Add unit test to ensure the deserialized KeyValuePairLogEvent can be successfully serialized into nlohmann::json objects, and be identical with the objects before serialization.

[gibber9809]

Mostly looks good to me -- just a few minor changes that could help make this code easier to understand.

Per previous discussions this PR is focused on finishing a functional implementation, so discussion about performance is left out of this review.

[kirkrodrigues]

Some renaming suggestions to be more explicit and avoid confusion between the schema tree node and corresponding JSON object. We can discuss it if you think it's too explicit, but I don't think the long names make the code significantly less readable.

[kirkrodrigues]

For the PR title, how about:

ffi: Add support for serializing a KeyValuePairLogEvent as a nlohmann::json object.