Add RestoreDocument API

[hackerwins]

What this PR does / why we need it:

In this PR, I implemented a logic to replace the contents of a document with the previous contents.

The summary logic is as follows:

1. Create a new change by copying contents from previous changes to restore the document(In the figure below, C5 is created by copying the contents from C2).
2. After applying the new change(has previous contents), concurrent changes should be rejected from the server. In the figure below, C4' C5' should be rejected.

The approaches to creating a new change with the previous contents are:

Approach 1: Creating change using snapshot operation

• Logic:
  • Deepcopy the root element then create a Snapshot Operation with the root element
• Pros:
  • The size of the change is relatively small. Because it has only one Snapshot Operation.
• Cons:
  • Conflict can occur if we apply operations created logically before the snapshot.
  • We need to add a new operation called Snapshot Operation.

Approach 2: Creating change using existing operations

• Logic:
  • Delete all keys from the root object.
  • Create individual editing operations while traversing snapshots
• Pros
  • Conflict cannot occur even if we apply operations created logically before the snapshot.
  • There is no need to introduce a separate operation.
• Cons:
  • The size of the change is relatively large and the execution complexity of the application is higher than that of the snapshot.

I implemented the first approach in this PR.

Which issue(s) this PR fixes:

Address #52

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

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

[codecov]

Codecov Report

Merging #303 (d0ce917) into main (9e153aa) will decrease coverage by 1.52%.
The diff coverage is 12.26%.

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   48.39%   46.86%   -1.53%     
==========================================
  Files          59       59              
  Lines        4821     4982     +161     
==========================================
+ Hits         2333     2335       +2     
- Misses       2229     2388     +159     
  Partials      259      259              

Impacted Files	Coverage Δ
api/converter/to_pb.go	79.18% <0.00%> (-3.38%)	⬇️
client/client.go	9.62% <0.00%> (-0.87%)	⬇️
pkg/document/document.go	43.67% <0.00%> (-13.04%)	⬇️
pkg/document/internal_document.go	20.63% <0.00%> (-0.68%)	⬇️
pkg/document/json/array.go	33.33% <0.00%> (-3.04%)	⬇️
pkg/document/json/counter.go	59.80% <0.00%> (-1.82%)	⬇️
pkg/document/json/object.go	21.05% <0.00%> (-2.48%)	⬇️
pkg/document/json/primitive.go	94.02% <0.00%> (-2.16%)	⬇️
pkg/document/json/rich_text.go	55.55% <0.00%> (-6.95%)	⬇️
pkg/document/json/text.go	43.66% <0.00%> (-7.16%)	⬇️
... and 8 more

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