refactor: Misc tidyup of SMS implementation

[SMadani]

This PR improves test coverage of the SMS API and deprecates classes intended for removal.

[codecov-commenter]

Codecov Report

Merging #513 (bbd3638) into main (730a1a4) will increase coverage by 0.54%.
The diff coverage is 98.07%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #513      +/-   ##
============================================
+ Coverage     95.29%   95.84%   +0.54%     
- Complexity     2933     2974      +41     
============================================
  Files           428      430       +2     
  Lines          6976     6999      +23     
  Branches        403      403              
============================================
+ Hits           6648     6708      +60     
+ Misses          198      165      -33     
+ Partials        130      126       -4     

Files	Coverage Δ
...m/vonage/client/auth/hashutils/AbstractHasher.java	83.33% <ø> (ø)
...om/vonage/client/auth/hashutils/HmacMd5Hasher.java	85.71% <ø> (ø)
...m/vonage/client/auth/hashutils/HmacSha1Hasher.java	85.71% <ø> (ø)
...vonage/client/auth/hashutils/HmacSha256Hasher.java	85.71% <ø> (ø)
...vonage/client/auth/hashutils/HmacSha512Hasher.java	85.71% <ø> (ø)
...va/com/vonage/client/auth/hashutils/Md5Hasher.java	100.00% <ø> (ø)
.../java/com/vonage/client/incoming/MessageEvent.java	88.46% <ø> (ø)
...n/java/com/vonage/client/incoming/MessageType.java	100.00% <100.00%> (ø)
...n/java/com/vonage/client/incoming/NotifyEvent.java	100.00% <ø> (ø)
src/main/java/com/vonage/client/sms/HexUtil.java	91.89% <100.00%> (-2.71%)	⬇️
... and 11 more