Use pre-defined proxy if RequestMeta contains proxyId

[reki2000]

Proof-of-concept implementation for #180.

Client API for newRequest is changed - need additional meta parameter for 'proxyId' (optional). Clients should set this value by using 'X-OCTOPARTS-PROXY-ID' from HTTP header. This value is also passed to service provider so that the service provider pass it to other subsequencial request via Octoparts.

[lloydmeta]

Nice job putting together a PoC 👍

A few thoughts:

• Can you move this to a fork (see Contributor guidelines)
• Proxy should probably become it's own class and persisted into the DB (likely in its own separate table) rather than be something that needs to be set via the config file. That way no new code or resets need to happen for adding new proxies (goes along with the design philosophy of Octoparts)
  • An HttpPartConfig can then have a Option[ProxyConfig] or something similar
• I think asking for an invalid proxy should not fail silently. For example, asking for an invalid part id (one that does not exist) prevents the part from even working at all; asking for a proxy by an invalid id, it follows, should also fail, otherwise there could be lots of confusion

@xevix would like to hear your thoughts as well.

[lloydmeta]

I don't think we need a case here.

We should also make sure to test that the header is being properly forwarded (and left out if no proxy) in subsequent requests to backends from Octoparts.

Also, since this method is called buildTracingHeaders, we should probably do proxy id header adding in another method or rename this method.

[reki2000]

I made another PR from my fork as commedted by @lloydmeta. Closing this PR.

[lloydmeta]

Closing for realsies.