Feat: add install-ext make target for sqlsrv dev setup

[Rom1-B]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes #N/A
• Adds a make install-ext target that installs the sqlsrv PHP extension (PECL) inside the dev container.
• Documents the one-time setup step in README so contributors can get the plugin running after a container rebuild.