fix: replace superagent-proxy with direct use of proxy-agent (#389)

## Which problem is this PR solving?

- Resolves #380 

## Short description of the changes

This is the equivalent behavior of the previous agent configuration. If
a proxy URL is provided in libhoney config, a single ProxyAgent will be
created with that proxy URL set for all connections. No nuanced lookup
of proxy config from the environment based on target URL protocol.

[proxy-agent is a hefty
import](https://bundlephobia.com/package/[email protected]), but we were
importing an early edition of it
([v5.0.0](https://bundlephobia.com/package/[email protected])) already
via superagent-proxy. 

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Co-authored-by: JamieDanielson <[email protected]>