Add conversations API

[andregasser]

https://docs.joinmastodon.org/methods/conversations/

Closes #196

[codecov]

Codecov Report

Merging #235 (0b742c6) into master (78d11a7) will increase coverage by 0.00%.
The diff coverage is 47.91%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #235   +/-   ##
=========================================
  Coverage     47.43%   47.44%           
- Complexity      298      307    +9     
=========================================
  Files            87       90    +3     
  Lines          2650     2698   +48     
  Branches        146      146           
=========================================
+ Hits           1257     1280   +23     
- Misses         1337     1362   +25     
  Partials         56       56           

Impacted Files	Coverage Δ
.../kotlin/social/bigbone/rx/RxConversationMethods.kt	0.00% <0.00%> (ø)
...n/social/bigbone/api/method/ConversationMethods.kt	92.30% <92.30%> (ø)
...e/src/main/kotlin/social/bigbone/MastodonClient.kt	31.86% <100.00%> (+0.25%)	⬆️
...n/kotlin/social/bigbone/api/entity/Conversation.kt	100.00% <100.00%> (ø)

[bocops]

There are some differences in entity definition vs. what we're doing elsewhere.

Additionally, one difference in the delete method, although this way could be preferable to what we're currently doing elsewhere - not completely sure about that.

[bocops]

Looks good! :)