Stop using Tomcat Base64 util

[nagyesta]

Issue: #1293

Description

• Removes explicit dependency on Tomcat's Base64 implementation
• Adds new tests to verify that the encoding/decoding works

Entry point

Checklists

• I have rebased my branch
• My commit message is meaningful
• The commit messages use semantic versioning: {major}, {minor} or {patch}
• The changes are focusing on the issue at hand
• I have maintained or increased test coverage

Notes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.89%. Comparing base (e538cc5) to head (db84006).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1294   +/-   ##
=========================================
  Coverage     98.89%   98.89%           
- Complexity     2030     2032    +2     
=========================================
  Files           239      239           
  Lines          4963     4965    +2     
  Branches        196      196           
=========================================
+ Hits           4908     4910    +2     
  Misses           42       42           
  Partials         13       13           

Flag	Coverage Δ
app	99.37% <100.00%> (+<0.01%)	⬆️
client	94.00% <ø> (ø)
testcontainers	96.03% <ø> (ø)

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