feat: Add spice user agent, update for clippy rules

[peasee]

🗣 Description

• Updates all code to address existing clippy pedantic rules
• Adds get_user_agent to retrieve user agent for a specific system type, including a regex test for checking the contents of the string
• Updates the test workflow to be more robust
• Adds x-spice-user-agent header to Flight client

🔨 Related Issues

• Closes task(rs): Add x-spice-user-agent header to Runtime requests #43

[github-advanced-security]

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