BUG #17045: 14 Beta Tighten up allowed names for custom GUC parameters breaks PostgREST

The following bug has been logged on the website:

Bug reference:      17045
Logged by:          Robert Sosinski
Email address:      robert.sosinski@reactive.io
PostgreSQL version: 14beta1
Operating system:   All
Description:

Recently (April 7th 2020) a new change was made to the PostgreSQL 14 beta
that changes allowed names for custom GUC keys. Please see the following:

1. The code change:
https://github.com/postgres/postgres/commit/3db826bd55cd1df0dd8c3d811f8e5b936d7ba1e4
2. Initial question about valid GUC keys:
https://www.postgresql.org/message-id/flat/20210209144059.GA21360%40depesz.com
3. More conversation about what valid GUC keys should be:
https://www.postgresql-archive.org/Tightening-up-allowed-custom-GUC-names-td6178392.html

This change allows there to only be one `.` in the GUC key.  The problem is
that PostgreSQL uses multiple `.` in GUC keys to nest parameters, such as
`request.jwt.claim.role`.  As such this would be a change that significantly
breaks PostgREST.

You can see the conversation at PostgREST here:
https://github.com/PostgREST/postgrest/issues/1857

It seems this change was mainly targeting special characters such as `-` and
`=`, could this be updated to allow multiple `.` characters for GUC keys as
well? There does not seem there is any issues with multiple `.` characters
in GUC keys.

Thanks!