fix: Server startup issue for external postgres (#38019)

## Description
We introduced the auth for embedded Postgres, but in pg-utils we missed
on updating the pg_isready condition to resort to db host in case of
external postgres url. This caused the pg_isready to never pass when
user configure external postgres url as a persistence DB in Appsmith.
This PR checks if the local postgres url is configured and accordingly
modifies the pg_isready command to target the embedded host vs external
host.

Why it did not get caught in our testing?
We are still investigating this but in local for some reason even if we
opt for external Postgres DB embedded Postgres process is also
initiating which runs the pg_isready against the embedded Postgres and
startup goes through.

Fixes #`Issue Number`  
_or_  
Fixes `Issue URL`
> [!WARNING]  
> _If no issue exists, please create an issue first, and check with the
maintainers if the issue is valid._

## Automation

/ok-to-test tags=""

### 🔍 Cypress test results
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> [!CAUTION]  
> If you modify the content in this section, you are likely to disrupt
the CI result for your PR.

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## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [ ] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced PostgreSQL connection handling with dynamic checks for local
or remote instances.
- Improved flexibility in connection strategy for better database
accessibility.

- **Bug Fixes**
- Updated connection commands to ensure proper usage of the new host
proxy variable.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

deploy/docker/fs/opt/appsmith/pg-utils.sh

@@ -18,9 +18,14 @@ waitForPostgresAvailability() {
     MAX_RETRIES=50
     RETRYSECONDS=10
     retry_count=0
-    local unix_socket_directory=$(get_unix_socket_directory "$POSTGRES_DB_PATH")
+    local host_proxy
+    if [[ "$PG_DB_HOST" == "localhost" || "$PG_DB_HOST" == "127.0.0.1" ]]; then
+      host_proxy=$(get_unix_socket_directory)
+    else
+      host_proxy=$PG_DB_HOST
+    fi
     while true; do
-      su postgres -c "pg_isready -h $unix_socket_directory -p '${PG_DB_PORT}'"
+      su postgres -c "pg_isready -h $host_proxy -p '${PG_DB_PORT}'"
       status=$?
 
       case $status in