Added sample for DLP : Inspect a string for sensitive data, excluding a custom substring

[sameer-crest]

Implemented sample for Inspect a string for sensitive data, excluding a custom substring
Added test cases for the same

Reference: https://cloud.google.com/dlp/docs/samples/dlp-inspect-string-custom-excluding-substring

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• dlp/src/inspect_string_custom_excluding_substring.php:27, tag dlp_inspect_string_custom_excluding_substring

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