PG-1603 Make pg_basebackup work with encrypted WAL

[dAdAbird]

When WAL is streamed during the backup (default mode), it comes in unencrypted. But we need keys to encrypt it. For now, we expect that the user would put pg_tde dir containing the 1664_key and 1664_providers into the destination directory before starting the backup. And we encrypt streamed WAL according to the internal keys. No pg_tde dir means no streamed WAL encryption.

Why I created TDEXLogCryptBuffer():
stream->walmethod->ops is an interface and there are at least four implementation of the write() method which accepts a buffer as argument. I thought about using TDEXLogSmgrInitWriteReuseKey() + pipe + xlog_smgr->seg_write(), but my experiments show that it is possible for pg_basebackup to start a stream from a couple of "wal keys back", therefore reusing of the last key won't work.

[dAdAbird]

Supersedes #473

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.22%. Comparing base (87c55e6) to head (cfda536).
⚠️ Report is 4 commits behind head on TDE_REL_17_STABLE.

❌ Your project status has failed because the head coverage (82.22%) 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     #513      +/-   ##
=====================================================
+ Coverage              82.00%   82.22%   +0.22%     
=====================================================
  Files                     24       25       +1     
  Lines                   3162     3179      +17     
  Branches                 514      515       +1     
=====================================================
+ Hits                    2593     2614      +21     
+ Misses                   460      456       -4     
  Partials                 109      109              

Components	Coverage Δ
access	83.57% <93.61%> (+0.69%)	⬆️
catalog	87.61% <ø> (ø)
common	77.77% <ø> (ø)
encryption	72.97% <ø> (ø)
keyring	73.21% <ø> (ø)
src	94.15% <ø> (ø)
smgr	95.29% <ø> (ø)
transam	∅ <ø> (∅)

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