feat(argv): Support span streaming

[sentrivana]

Description

The argv integration currently sets sys.argv on events, including transactions. In span streaming, this corresponds to setting an attribute on the segment span. The attr was added to Sentry conventions in getsentry/sentry-conventions@420bab8.

I also split up the span streaming transport format test into:

1. a test that only checks the transport format (not so much the actual data, just that it's formatted correctly) and
2. a test that checks that the default set of attributes we expect is set.

Issues

Closes #5998
Closes https://linear.app/getsentry/issue/PY-2300/migrate-argv-to-span-first

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[linear-code]

PY-2300 Migrate argv to span first

[github-actions]

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