Rocky8 support + CCP_PG_EXTENSIONS env for easy install additional extensions

[pere3]

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)
currently there is no way to build yourself a crunchy-containers if you are not a crunchy data customer (because ubi8 images requires licences) and centos8 support is broken since centos8 went EOL (December 31st, 2021)

there is also no easy way to customize images with additional pg_extensions you may need
#1335
https://github.com/CrunchyData/postgres-operator/issues/2016

What is the new behavior (if this is a feature change)?
i've added rocky8 support as it currently stands as centos replacement and also introduced CCP_PG_EXTENSIONS env which can be used to easily add list of extensions for your build

export CCP_PG_EXTENSIONS="pg_qualstats_$CCP_PGVERSION hypopg_$CCP_PGVERSION powa_$CCP_PGVERSION"

there is also official postgres repo added for community builds

Other information:
there is no way for me to tests this builds on PG11 and PG12, as this versions is not available at https://repo.developers.crunchydata.com/, but PG13 and PG14 build just fine

Successfully tagged crunchydata/crunchy-base:rocky8-13.8-5.2.0-0
Successfully tagged crunchydata/crunchy-postgres:rocky8-13.8-5.2.0-0
Successfully tagged crunchydata/crunchy-pgbackrest:rocky8-13.8-5.2.0-0

Successfully tagged crunchydata/crunchy-base:rocky8-14.5-5.2.0-0
Successfully tagged crunchydata/crunchy-postgres:rocky8-14.5-5.2.0-0
Successfully tagged crunchydata/crunchy-pgbackrest:rocky8-14.5-5.2.0-0

also only version 2.36 of pgbackrest is available here and 4.20 for pgadmin4

[jelmer]

I've tested this PR and can confirm it works well. I did need to explicitly specify the registry URL for rocky8 - see pere3#1