Add support for config file

[Maelkum]

This PR adds support for loading configuration options from a config file besides using the CLI arguments.

It's possible to use both side by side, and in that case the CLI options take precedence and override the config file options. Default values for CLI flags are used where applicable.

Values are determined by (less to more important):

1. Default CLI flags
2. Config file options
3. CLI flags

This might not be the final version, it might evolve further if it turns out to be too clunky for use from the dev side (user is fine IMO).

Complication comes from the fact that the CLI flags are flat (e.g. --address 127.0.0.1 --port 9000 --runtime-path ~/blockless/runtime), compared to the file which is structured. Currently the config file has four sections - log, connectivity, worker and head. In the example above, the address and port options live in the connectivity section, whereas the runtime-path lives in the worker section.

I didn't want to go the typical route of the group being embedded in the CLI flag (--connectivity-address, --connectivity-port, --worker-runtime-path) as it's pretty noisy. Instead, we define the group in which the flag lives and we map values to sections in code. This mapping is done in flags.go.

Example of a config file:

role: worker
concurrency: 10
workspace: /tmp/workspace
attributes: false

log:
  level: debug

connectivity:
  address: 127.0.0.1
  port: 9000
  private-key: ~/.b7s/path/to/priv/key.bin
  websocket: true


worker:
  runtime-path: ~/.local/blockless-runtime/bin
  cpu-percentage-limit: 0.8

For a more detailed reference of the config file, I added example.yaml.

[dmikey]

love this change!