PProf compatible endpoints

I am trying to use parca to scrape profiles from a long-running Julia process, and it support the pprof/go debug API

My config is:
```
scrape_configs:
- job_name: "parca"
scrape_interval: "10s"
scrape_timeout: "60s"
static_configs:
- targets: [ '127.0.0.1:16825' ]
profiling_config:
pprof_config:
julia:
enabled: true
path: '/profile'
delta: true
```

and the last entry in the screenshot corresponds to the `pprof_config`. The `delta: true` adds the seconds option to the call.

So if we rename / or allow seconds in place of duration, then with some fixes to our pprof format
https://github.com/JuliaPerf/PProf.jl/pull/106

We should be able to support pprof/go compatible tools.