Add JsonSerializable XXStruct classes

[7hong13]

What this PR does / why we need it?

• Cleaned up test-related terminology by renaming getStructureAsString() to toTestString().
• Introduced JsonSerializableStruct
  • TimeTicket has a JSON serialization issue due to its lamport property being of type Long, which is not directly supported in JSON serialization. It can be problematic when CrdtTreePos and RgaTreeSplitPos, which have a createdAt property, need be serialized into JSON strings and passed through Presence between platforms for selection handling.
  • To properly serialize TimeTicket, its lamport type should be String, so I added TimeTicketStruct and other XXStruct data types based on the JS-SDK implementation. The postfix Struct is used to explicitly indicate that these classes are exclusively designed for JSON serialization.

Any background context you want to provide?

This PR only includes works released in v0.4.4.

What are the relevant tickets?

• Use TreeRangeStruct to represent tree selection yorkie-js-sdk#561
• Cleanup of test-related terminology  yorkie-js-sdk#562

Fixes #

Checklist

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #129 (462dc47) into main (ddb5aa3) will decrease coverage by 0.65%.
Report is 2 commits behind head on main.
The diff coverage is 68.49%.

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   83.70%   83.05%   -0.65%     
==========================================
  Files          62       63       +1     
  Lines        3620     3654      +34     
  Branches      536      534       -2     
==========================================
+ Hits         3030     3035       +5     
- Misses        297      325      +28     
- Partials      293      294       +1     

Files Changed	Coverage Δ
...c/main/kotlin/dev/yorkie/document/json/JsonTree.kt	79.56% <37.50%> (-0.14%)	⬇️
...tlin/dev/yorkie/document/JsonSerializableStruct.kt	52.94% <52.94%> (ø)
...c/main/kotlin/dev/yorkie/document/json/JsonText.kt	75.78% <54.54%> (-3.34%)	⬇️
...n/dev/yorkie/document/operation/SelectOperation.kt	33.33% <66.66%> (-27.78%)	⬇️
...in/kotlin/dev/yorkie/document/crdt/RgaTreeSplit.kt	79.77% <71.42%> (-0.46%)	⬇️
...src/main/kotlin/dev/yorkie/api/ElementConverter.kt	76.89% <100.00%> (ø)
...c/main/kotlin/dev/yorkie/document/crdt/CrdtText.kt	89.70% <100.00%> (-2.95%)	⬇️
...c/main/kotlin/dev/yorkie/document/crdt/CrdtTree.kt	75.27% <100.00%> (+0.36%)	⬆️
...c/main/kotlin/dev/yorkie/document/crdt/TextInfo.kt	87.87% <100.00%> (ø)
...lin/dev/yorkie/document/operation/EditOperation.kt	77.14% <100.00%> (-2.86%)	⬇️
... and 2 more

... and 3 files with indirect coverage changes