[Bug]: Dependency extensions may resolve to wrong distribution/pgMajor during testingValues generation

[NiccoloFei]

When generating TestingValues for an extension with required_extensions, each dependent extension will always wrongly point to its defaultImage (e.g with default pgMajor + distro) instead of following the
specification of the parent extension.

Reproducer

As of today, we don't have any extension yet with one or more required_extensions that's been onboarded.
So, to reproduce this issue:

1. Set a random extension dependency for an extension. For example, set
   required_extensions = [ "pgvector" ] in the metadata of timescaledb-oss
2. Run task generate-values using a non-default distribution (e.g bookworm):

       task generate-values \
         TARGET=timescaledb-oss \
         EXTENSION_IMAGE=ghcr.io/cloudnative-pg/timescaledb-oss:2.27.1-18-bookworm
   

3. Notice the mismatch inside timescaledb-oss/values.yaml:

    extensions:
           - name: timescaledb-oss
             image:
               reference: ghcr.io/cloudnative-pg/timescaledb-oss:2.27.1-18-bookworm
           - name: pgvector
             image:
               reference: ghcr.io/cloudnative-pg/pgvector:0.8.2-18-trixie