[Slim] Add PHP CodeSniffer config template

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Recommended way to configure PHP CodeSniffer:

If you run PHP_CodeSniffer without specifying a coding standard, PHP_CodeSniffer will look in the current directory, and all parent directories, for a file called either .phpcs.xml, phpcs.xml, .phpcs.xml.dist, or phpcs.xml.dist. If found, configuration information will be read from this file, including the files to check, the coding standard to use, and any command line arguments to apply.

Note: If multiple default configuration files are found, PHP_CodeSniffer will select one using the following order: .phpcs.xml, phpcs.xml, .phpcs.xml.dist, phpcs.xml.dist

Ref: Using a Default Configuration File

Generator cli option phpcsStandard is deprecated as not necessary anymore.

Upgrade note

Do not use phpcsStandard cli option during server stubs generation. Override phpcs.xml.dist in root folder with phpcs.xml to change config.