Enabling options for hackney

[h4cc]

This library is using hackney, but there is no predefined way to provide additional options to hackney. These could include:

• Pool configuration
• Proxy settings

Currently sentry is using the default hackney pool, which will lead to failures when sending to sentry, when the pool is empty.

[mitchellhenke]

Thanks for creating this issue, I should be able to take care of it this week! 🙂