pgsql: Add pg_alterckey utility to change the cluster key

Add pg_alterckey utility to change the cluster key

This can change the key that encrypts the data encryption keys used for
cluster file encryption.

Discussion: https://postgr.es/m/20201202213814.GG20285@momjian.us

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/62afb42a7f9f533efc6c19f462c3a848fa4ddb63

Modified Files
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doc/src/sgml/ref/pg_alterkey.sgml   | 186 ++++++++++
src/bin/Makefile                    |   1 +
src/bin/pg_alterckey/.gitignore     |   1 +
src/bin/pg_alterckey/Makefile       |  44 +++
src/bin/pg_alterckey/pg_alterckey.c | 693 ++++++++++++++++++++++++++++++++++++
5 files changed, 925 insertions(+)