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

❌ 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).
⚠️ Report is 191 commits behind head on main.

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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