Add Presence for Client.Options

[7hong13]

What this PR does / why we need it?

Currently, users set Client.Options.presence using PresenceInfo directly, but I think it is not advisable for users to manipulate or access PresenceInfo.clock directly. (iOS, JS SDKs are not exposing it to users.)
To address this, I have made clock property internally visible only and added Presence value class.
I also added a test on accessing peer presences based on this PR.

Any background context you want to provide?

What are the relevant tickets?

Fixes #

Checklist

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

[codecov]

Codecov Report

Merging #108 (6e31411) into main (707e3dc) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##               main     #108      +/-   ##
============================================
- Coverage     78.57%   78.54%   -0.03%     
  Complexity      545      545              
============================================
  Files            53       53              
  Lines          2619     2620       +1     
  Branches        357      357              
============================================
  Hits           2058     2058              
- Misses          367      368       +1     
  Partials        194      194              

Impacted Files	Coverage Δ
yorkie/src/main/kotlin/dev/yorkie/core/Client.kt	77.62% <50.00%> (ø)
yorkie/src/main/kotlin/dev/yorkie/core/Peers.kt	84.61% <50.00%> (-7.06%)	⬇️