Add config option for setting the app's base url

[geoah]

This PR adds a config option to add a base meta tag in the pyroscope webapp.

The rationalle behind this is to allow the app to be served as a path behind a reverse proxy (ie. https://some.host/pyroscope/)

This can technically be done via the EXTRA_METADATA env var, but requires a path to a file which makes it a bit more cumbersome. (And the render path would need to be made relative still.)

---

In order to test this out you can do so in a local docker-compose with caddy.

You'll need a docker-compose.yml:

version: "3.8"

services:
  caddy:
    image: caddy
    container_name: caddy
    ports:
      - 8080:8080
    volumes:
      - ./Caddyfile:/etc/caddy/Caddyfile:ro
  
  pyroscope:
    image: pyroscope/pyroscope:dev
    container_name: pyroscope
    ports:
      - 4040:4040
    environment:
      - PYROSCOPE_BASE_URL
    command: server

and a Caddyfile:

{
    http_port 8080
    auto_https off
}

http://localhost {
    redir /pyroscope /pyroscope/
    route /pyroscope* {
        uri strip_prefix pyroscope
        reverse_proxy pyroscope:4040
    }
}

Running docker-compose up and visiting http://localhost:8080/pyroscope/ should work fine (assuming you've first build the container using make build-dev).

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[petethepig]

Looks good, I tested with the docker-compose provided

Thank you @geoah !