Replace hex.EncodeTostring with ActorID.key in Text and RichText

[hackerwins]

What this PR does / why we need it:

Replace hex.EncodeTostring with ActorID.key in Text and RichText.

Before:

After:

Which issue(s) this PR fixes:

Address #251

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 #255 (dcb1d1c) into main (dc3b660) will decrease coverage by 0.15%.
The diff coverage is 15.62%.

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
- Coverage   49.95%   49.79%   -0.16%     
==========================================
  Files          54       54              
  Lines        4422     4426       +4     
==========================================
- Hits         2209     2204       -5     
- Misses       1996     2005       +9     
  Partials      217      217              

Impacted Files	Coverage Δ
client/client.go	11.46% <0.00%> (ø)
pkg/document/json/rich_text.go	51.02% <0.00%> (ø)
pkg/document/json/text.go	42.46% <0.00%> (ø)
pkg/document/time/actor_id.go	55.17% <0.00%> (-6.37%)	⬇️
yorkie/backend/sync/memory/pubsub.go	48.35% <0.00%> (ø)
pkg/document/time/ticket.go	86.04% <50.00%> (-13.96%)	⬇️
pkg/document/json/rga_tree_split.go	73.87% <75.00%> (ø)

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[zeikar]

LGTM 👍