RUM-2446: Open text masking classes for extension

[jonathanmos]

What does this PR do?

Modify MaskTextViewMapper and MaskInputTextViewMapper to be open for extension.

Motivation

Necessary for React Native Session Replay, in order to support masking for text elements.

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

Merging #1757 (e9a2463) into develop (bece8a1) will decrease coverage by 0.05%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1757      +/-   ##
===========================================
- Coverage    83.34%   83.29%   -0.05%     
===========================================
  Files          468      468              
  Lines        16304    16301       -3     
  Branches      2431     2430       -1     
===========================================
- Hits         13587    13577      -10     
- Misses        2050     2057       +7     
  Partials       667      667              

Files	Coverage Δ
...nternal/recorder/mapper/MaskInputTextViewMapper.kt	80.00% <ø> (ø)
...lay/internal/recorder/mapper/MaskTextViewMapper.kt	100.00% <ø> (ø)

... and 23 files with indirect coverage changes