PG-1857 Do not start basebackup w/o WAL keys

[dAdAbird]

To check if everyone's on board with the idea.
This requires small changes in the backend and basebackup protocol. But it is more reliable - the backup won't start at all if the source server has wal_keys but the destination doesn't.
Another, simpler approach would be to check during the file copy if we stumble upon pg_tde/wal_keys and, if there is no such file in the destination, then we abort the backup. It would require changes only in bin/pg_basebackup. But it can expose unencrypted WAL files in the backup directory (the backup might take who knows how long until we encounter pg_tde/wal_keys).

The code needs improvements - get rid off hardcoded strings etc. Plus we might want to check on the server side if the last WAL key is encrypted instead of just existence of the file.

Also needs tests

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.20%. Comparing base (b2bb77c) to head (d986221).
⚠️ Report is 2 commits behind head on TDE_REL_17_STABLE.

❌ Your project status has failed because the head coverage (82.20%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@                Coverage Diff                 @@
##           TDE_REL_17_STABLE     #527   +/-   ##
==================================================
  Coverage              82.19%   82.20%           
==================================================
  Files                     25       25           
  Lines                   3174     3175    +1     
  Branches                 515      516    +1     
==================================================
+ Hits                    2609     2610    +1     
  Misses                   456      456           
  Partials                 109      109           

Components	Coverage Δ
access	83.47% <ø> (ø)
catalog	87.60% <ø> (ø)
common	77.77% <ø> (ø)
encryption	72.97% <ø> (ø)
keyring	73.21% <ø> (ø)
src	94.15% <ø> (ø)
smgr	95.32% <ø> (+0.02%)	⬆️
transam	∅ <ø> (∅)

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