Add secret config store

[dominikschulz]

This PR adds an encrypted config store for credentials needed to access backends (e.g. Consul or Vault Tokens).

[codecov]

Codecov Report

Merging #708 into master will decrease coverage by 0.87%.
The diff coverage is 41.76%.

@@            Coverage Diff             @@
##           master     #708      +/-   ##
==========================================
- Coverage   65.49%   64.62%   -0.87%     
==========================================
  Files         152      156       +4     
  Lines        8692     8889     +197     
==========================================
+ Hits         5692     5744      +52     
- Misses       2328     2459     +131     
- Partials      672      686      +14

Impacted Files	Coverage Δ
app.go	57.14% <ø> (ø)	⬆️
backend/storage.go	100% <ø> (ø)	⬆️
backend/crypto/xc/xc.go	57.5% <0%> (ø)	⬆️
store/root/store.go	45% <0%> (-9.35%)	⬇️
store/root/git.go	0% <0%> (ø)	⬆️
store/sub/init.go	20% <0%> (-1.43%)	⬇️
store/root/gpg.go	0% <0%> (ø)	⬆️
config/io.go	72.73% <100%> (+0.36%)	⬆️
action/action.go	100% <100%> (ø)	⬆️
store/root/mount.go	55.43% <100%> (ø)	⬆️
... and 20 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d8a229e...d5b633f. Read the comment docs.