Encode traces into provided buffer.

[mstyura]

Fixes #
Design discussion issue (if applicable) #

Changes

Refactor code to prepare to work with pre-allocated and re-used buffer to serialize data exported to DD.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 9 lines in your changes missing coverage. Please review.

Project coverage is 53.7%. Comparing base (5308844) to head (99dfc3c).

Files with missing lines	Patch %	Lines
opentelemetry-datadog/src/exporter/mod.rs	0.0%	4 Missing ⚠️
opentelemetry-datadog/src/exporter/model/v05.rs	94.1%	4 Missing ⚠️
opentelemetry-datadog/src/exporter/model/v03.rs	97.3%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #162     +/-   ##
=======================================
+ Coverage   53.6%   53.7%   +0.1%     
=======================================
  Files         43      43             
  Lines       6166    6189     +23     
=======================================
+ Hits        3306    3327     +21     
- Misses      2860    2862      +2     

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