Implement remote configuration

[beniwohli]

• Implement API
• Implement regular update call
• Implement hot-reloading of configuration

see elastic/apm#76

[bmorelli25]

I'll be on vacation when you implement this feature. Here's some boilerplate documentation you can hopefully adapt to match your config style: elastic/apm#86. Also, don't forget to update the precedence list. Would be great if it included a link to the new configuration variable.

[beniwohli]

This has been implemented in #511, precedence list has been adapted in #548