add support for custom http request headers per servergroup

[urgerestraint]

Context

• Grafana Mimir and Cortex-Tenant both require the use of an X-Scope-OrgID header to specify for which TSDB tenant a read or write request is being made for (these may be concatenated with pipes between each tenant string); more here
• To support the query and remote read paths for Grafana Mimir, this change adds a configurable map of headers to be attached to each request for a given ServerGroup via config
• Addresses https://github.com/jacksontj/promxy/issues/643

Changes

• Add HTTPClientHeaders as a simple string map to ServerGroup config to match the middleware format
• On the custom RoundTripper, also mutate the request with the SG level headers on each request, after the middleware to respect proxy-headers

Fixes #643

[jacksontj]

Thanks for the contribution!