Add a system property for a max index size for PDF

Existing System Properties:

• atlassian.indexing.contentbody.maxsize - configures the max size to index for all attachments
• officeconnector.spreadsheet.xlsxmaxsize - does the same for Excel files
• officeconnector.textextract.word.docxmaxsize - works for Word files

I'm requesting that we add a property for PDF files specifically as indexing of PDF files has specific performance concerns that could be tuned directly with an attribute for that file type alone.