refactor(fdo): use openbao for key storage

[noaccOS]

What this PR does / why we need it:

Currently we're storing plaintext private keys on db for FDO, which is a big no no
The chosen solution is openbao!

Additional documentation e.g. usage docs, diagrams, reviewer notes, etc.:

---

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[codecov]

Codecov Report

❌ Patch coverage is 70.46980% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.68%. Comparing base (7234b63) to head (2c49d2a).

Files with missing lines	Patch %	Lines
.../astarte_fdo/lib/ownership_voucher/load_request.ex	82.66%	13 Missing ⚠️
...rte_secrets/lib/astarte_secrets/astarte_secrets.ex	0.00%	13 Missing ⚠️
...data_access/lib/astarte_data_access/fdo/queries.ex	8.33%	11 Missing ⚠️
libs/astarte_secrets/lib/astarte_secrets/core.ex	84.37%	5 Missing ⚠️
libs/astarte_fdo/lib/owner_onboarding.ex	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1905      +/-   ##
==========================================
+ Coverage   83.82%   84.68%   +0.85%     
==========================================
  Files         572      394     -178     
  Lines       10040     8331    -1709     
==========================================
- Hits         8416     7055    -1361     
+ Misses       1624     1276     -348     

Files with missing lines	Coverage Δ
...keeping/lib/astarte_housekeeping/realms/queries.ex	91.18% <ø> (+0.33%)	⬆️
...s/lib/astarte_data_access/fdo/ownership_voucher.ex	100.00% <100.00%> (ø)
..._access/lib/astarte_data_access/fdo/to2_session.ex	100.00% <ø> (ø)
..._fdo/lib/owner_onboarding/key_exchange_strategy.ex	100.00% <100.00%> (ø)
libs/astarte_fdo/lib/owner_onboarding/session.ex	84.74% <100.00%> (+1.10%)	⬆️
...ibs/astarte_fdo_core/lib/ownership_voucher/core.ex	29.41% <ø> (ø)
...e_fdo_core/lib/ownership_voucher/create_request.ex	92.85% <ø> (ø)
...ts/lib/astarte_secrets/owner_key_initialization.ex	100.00% <100.00%> (+100.00%)	⬆️
libs/astarte_fdo/lib/owner_onboarding.ex	75.90% <50.00%> (+1.15%)	⬆️
libs/astarte_secrets/lib/astarte_secrets/core.ex	92.00% <84.37%> (+24.12%)	⬆️
... and 3 more

... and 181 files with indirect coverage changes

Flag	Coverage Δ
astarte_appengine_api	85.68% <ø> (ø)
astarte_data_access	81.94% <15.38%> (-3.77%)	⬇️
astarte_data_updater_plant	87.13% <ø> (ø)
astarte_events	80.27% <ø> (ø)
astarte_fdo	80.87% <82.95%> (+5.54%)	⬆️
astarte_fdo_core	61.50% <ø> (+0.87%)	⬆️
astarte_generators	97.74% <ø> (ø)
astarte_housekeeping	85.67% <ø> (+0.14%)	⬆️
astarte_housekeeping_api	?
astarte_pairing	?
astarte_pairing_api	?
astarte_realm_management	87.66% <ø> (+0.07%)	⬆️
astarte_realm_management_api	?
astarte_rpc	88.23% <ø> (ø)
astarte_secrets	85.16% <62.50%> (+14.63%)	⬆️
astarte_trigger_engine	82.52% <ø> (ø)

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