feat: introduce abra config file and load abra dir from it (!419)

This is the first step to introduce a configuration file for abra. The config file must be named `abra.yaml` or àbra.yml`. abra look for the config file in the current directory and when not found traverses the directory tree up until it is found or the home/root directory is reached.

For now there is only one setting that is made configurable: `abraDir`. The new logic for setting the abra dir is the following:
1. lookup `$ABRA_DIR` env
2. look for config file and take value from there
3. `$HOME/.abra` as fallback

See coop-cloud/organising#303.

Reviewed-on: coop-cloud/abra#419
Reviewed-by: decentral1se <decentral1se@noreply.git.coopcloud.tech>
Co-authored-by: p4u1 <p4u1_f4u1@riseup.net>
Co-committed-by: p4u1 <p4u1_f4u1@riseup.net>

pkg/config/testdata/abraconfig1/abra.yaml

@@ -0,0 +1 @@
+abraDir: foobar