Add getValueByPath() to the document

[humdrum]

What this PR does / why we need it:

• Add getValueByPath() to the document. (Allow specifying a topic when subscribing to a document yorkie-js-sdk#487)
• Modify API exposures to match JS API

Which issue(s) this PR fixes:

Fixes #

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 #86 (5b15cb8) into main (840e723) will increase coverage by 0.15%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   55.90%   56.05%   +0.15%     
==========================================
  Files          92       92              
  Lines       14637    14695      +58     
==========================================
+ Hits         8183     8238      +55     
- Misses       6454     6457       +3     

Impacted Files	Coverage Δ
Sources/Document/Change/Change.swift	100.00% <ø> (ø)
Sources/Document/Json/JSONObject.swift	86.40% <50.00%> (-0.66%)	⬇️
Sources/Util/LLRBTree.swift	85.99% <60.00%> (ø)
Sources/Document/Document.swift	50.36% <82.14%> (+3.59%)	⬆️
Tests/Unit/Document/DocumentTests.swift	98.59% <100.00%> (+0.03%)	⬆️
Tests/Unit/Document/JSONObjectTests.swift	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[chacha912]

Can we verify if the value is properly returned instead of checking if the array is not nil?
The getValueByPath() method is used to retrieve values, such as doc.getValueByPath('$.obj.num'), instead of using doc.getRoot().obj.num. Therefore, it would be better to verify if the value is properly returned.

(Refer to the example using getValueByPath in the js-sdk:
https://github.com/yorkie-team/yorkie-js-sdk/blob/395d052f2a6a3c1a5cef794a2e45b0312a63e945/public/multi.html#L169 )

[humdrum]

I added some Test code that verifies the value of getValueByPath() results.

[chacha912]

Thank you for the modification!
I have a question: Is it better to silently ignore the usage of a key containing . instead of explicitly throwing an error?

root["$$...hello"] = [] as [Any]

// The key name can't contain "."
XCTAssertTrue(root["$$...hello"] == nil)

[humdrum]

I will add an assert and modify it so that you can know explicitly whether the wrong value was used in the development.

[myupage]

Looks good to me :)