Refactor TEdge config (#155)

- Refactor TEdgeConfig

- Move configuration into `tedge_config` crate.

- Add `ConfigSetting` trait. Each "thing" that we want to make
  configurable will have it's own struct-type. For example `struct
  DeviceIdSetting` or `C8yUrlSetting`.

- The `ConfigSetting` trait does not provide any means of how to
  read/write/unset a configuration setting. It only provides the key,
  description and the value type of the setting, but no implementation.

- Use trait `ConfigSettingAccessor` to implemenent `query`, `update` and
  `unset` methods.

- `TEdgeConfigRepository` implements functionality to read/write a
  `TEdgeConfig`. The `TEdgeConfig` struct itself just holds data.

- Add `TEdgeConfigLocation` which holds all data that we need to resolve the defaults in
  TEdgeConfig, including the name of the config file (TODO: we might want to split these two apart). 
  The defaults differ depending on the location of the `tedge.toml`, whether it's located in `/etc` or a users `$HOME/.tedge`
  (or in tests).

- Only add the `tedge_config` crate. `tedge` is currently not modified
  to use the new API.

- Remove remaining `unwrap`s, also from tests

- No longer create intermediate directories upon saving configuration

# FOLLOW-UP TASKS

- Refactor the `tedge` crate to use `tedge_config`.
- Compute the `device.id` from the certificate stored in `device.cert.path`, if any.
- Ensure that the config is updated only by `tedge config set/unset`.
- Extend tedge_config_location to also manage the paths to the bridge configuration files.
- Update `ConfigRepository::store()` to properly set file ownership.

1 files changed, 1 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index f5e84120..c009a6bb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -3,6 +3,7 @@
 members = [
     "common/mqtt_client",
     "tedge",
+    "tedge_config",
     "mapper/tedge_mapper",
     "mapper/cumulocity/c8y_translator_lib",
 ]