added four new web-api methods

[kian2attari]

Summary

Added four methods to web-api:

• admin.conversations.restrictAccess.addGroup
• admin.conversations.restrictAccess.listGroups
• admin.conversations.restrictAccess.removeGroup
• conversations.mark

Linked issue: #1047

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[stevengill]

LGTM! Great work!

[codecov]

Codecov Report

Merging #1049 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1049   +/-   ##
=======================================
  Coverage   94.47%   94.47%           
=======================================
  Files          12       12           
  Lines         778      778           
  Branches      176      176           
=======================================
  Hits          735      735           
  Misses         14       14           
  Partials       29       29           

Flag	Coverage Δ
#eventsapi	89.67% <ø> (ø)
#interactivemessages	95.06% <ø> (ø)
#webapi	96.46% <ø> (ø)
#webhook	87.50% <ø> (ø)

Impacted Files	Coverage Δ
src/methods.ts	100.00% <0.00%> (ø)

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