Base64 cannot be decoded to KeyStore with Java client

[nagyesta]

Issue: #1295

Description

• Fixes encoder to generate same format as before
• Changes end-to-end tests to only accept the right format (and no longer accept Base64Mime)

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 (e15a9fb) to head (91a643c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1296   +/-   ##
=========================================
  Coverage     98.89%   98.89%           
  Complexity     2032     2032           
=========================================
  Files           239      239           
  Lines          4965     4965           
  Branches        196      196           
=========================================
  Hits           4910     4910           
  Misses           42       42           
  Partials         13       13           

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

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