refactor: begin to avoid autogenerated getters

[ee7]

Issue

#214

Description

To ease the transition to con4m v2, we want to reduce our usage of autogenerated getters in src/c4autoconf.nim.

The all-encompassing object that c4autoconf.nim defines is ChalkConfig, which currently contains:

type ChalkConfig* = ref object
  `@@attrscope@@`*: AttrScope
  [...]
  attestationConfig*: AttestationConfig
  [...]
  extractConfig*: ExtractConfig
  dockerConfig*: DockerConfig
  execConfig*: ExecConfig
  loadConfig*: LoadConfig
  envConfig*: EnvConfig
  srcConfig*: SrcConfig
  cloudProviderConfig*: CloudProviderConfig
  [...]

As a starting point, try to refactor away all accesses of the fields that end in Config (which are shown above).

That is, with this PR, the following command no longer returns any matches:

git grep 'chalkConfig\.\w*Config\.'

I'll split this work across a few PRs. Another PR will be for all the fields that are of a simple built-in type.

In con4m v2, we'll be using this proc:

proc lookup*[T](ctx: RuntimeState, key: string): Option[T]

like

runtime.lookup("foo.bar").get()

so this PR makes it easier for us to refactor to that later.

This PR adds a get and getOpt in config.nim that calls the get proc in con4m so we can write:

get[T](chalkConfig, "foo.bar")

or with the :T syntax (edit: this was vetoed):

chalkConfig.get[:T]("foo.bar")

Refs: #214

Testing

Hopefully the existing tests are sufficient.

[ee7]

@miki725 Please review :)