pgsql: psql: Add %i prompt escape to indicate hot standby status.

psql: Add %i prompt escape to indicate hot standby status.

This commit introduces a new prompt escape %i for psql, which shows
whether the connected server is operating in hot standby mode. It
expands to standby if the server reports in_hot_standby = on, and
primary otherwise.

This is useful for distinguishing standby servers from primary ones
at a glance, especially when working with multiple connections in
replicated environments where libpq's multi-host connection strings
are used.

Author: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Greg Sabino Mullane <htamfids(at)gmail(dot)com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133(at)gmail(dot)com>
Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Andreas Karlsson <andreas(at)proxel(dot)se>
Discussion: https://www.postgresql.org/message-id/flat/016f6738-f9a9-4e98-bb5a-e1e4b9591d46(at)uni-muenster(dot)de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/dddbbc253b923ef27f724c6abb5a6a39e1254d54

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 17 +++++++++++++++++
src/bin/psql/prompt.c | 18 ++++++++++++++++++
2 files changed, 35 insertions(+)