Fix: Add attachmentType to envelope ser/deser.

[marandaneto]

📜 Description

Fix: Add attachmentType to envelope ser/deser.

💡 Motivation and Context

Extract from https://github.com/getsentry/sentry-java/pull/1256/files

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[philipphofmann]

Just two minor comments. TIL. Didn't know the abbreviation ser/deser.

[codecov-commenter]

Codecov Report

Merging #1504 (90a3cda) into main (ac5797b) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1504      +/-   ##
============================================
+ Coverage     76.04%   76.12%   +0.07%     
- Complexity     1930     1934       +4     
============================================
  Files           191      191              
  Lines          6675     6696      +21     
  Branches        665      666       +1     
============================================
+ Hits           5076     5097      +21     
  Misses         1274     1274              
  Partials        325      325              

Impacted Files	Coverage Δ
sentry/src/main/java/io/sentry/Attachment.java	100.00% <100.00%> (ø)
...ry/src/main/java/io/sentry/SentryEnvelopeItem.java	88.78% <100.00%> (+0.10%)	⬆️
.../main/java/io/sentry/SentryEnvelopeItemHeader.java	92.59% <100.00%> (+1.68%)	⬆️
...ava/io/sentry/SentryEnvelopeItemHeaderAdapter.java	74.00% <100.00%> (+3.54%)	⬆️

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