feat: [WIP] Implement datadog v0.7 traces api

[RoyShulman]

Fixes

I've added an option to export traces using the v0.7 api endpoint to datadog. I needed this because as opposed to the v0.5 endpoint, the new one is able to export span links and events.
I've tested my code by using the test agent and added a link in the code as well so that others might find it useful. Also tested against a real datadog instance and everything seems to work correctly.

Design discussion issue (if applicable)

Since the v0.7 endpoint uses message pack encoding with string keys, it's possible to make the code nicer and probably safer by using rmp_serde. I wasn't sure if you wanted to pull in another dependency, so for now I implemented the serialization without it. Tell me if you want to change it :)

Changes

All changes are contained in the opentelemetry-datadog crate, mostly in the opentelemetry-datadog/src/exporter/model/v07.rs file.

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)

[linux-foundation-easycla]

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