feat(ExifTool): add capability to overwrite metadata of the PDF generated (#776)

* add new metadata functions to pdf engine interface

* add exiftool module with relevant test cases

* use exiftool to overwrite metadata in libreoffice

* use exiftool to overwrite metadata in chromium

* fix linter issues

* fix more linter issues

* more test cases for better coverage

* remove utils

* minor changes

* remove metadata from pdfoptions

* correct indentation

* read/write metadata one file at a time.

build/Dockerfile

@@ -171,12 +171,13 @@ RUN \
     echo '#!/bin/bash\n\nexec java -jar /usr/bin/pdftk-all.jar "$@"' > /usr/bin/pdftk && \
     chmod +x /usr/bin/pdftk &&\
     apt-get update -qq &&\
-    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends qpdf &&\
+    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends qpdf exiftool &&\
     # See https://github.com/nextcloud/docker/issues/380.
     mkdir -p /usr/share/man/man1 &&\
     # Verify installations.
     pdftk --version &&\
     qpdf --version &&\
+    exiftool --version &&\
     # Cleanup.
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
@@ -195,6 +196,7 @@ ENV LIBREOFFICE_BIN_PATH /usr/lib/libreoffice/program/soffice.bin
 ENV UNOCONVERTER_BIN_PATH /usr/bin/unoconverter
 ENV PDFTK_BIN_PATH /usr/bin/pdftk
 ENV QPDF_BIN_PATH /usr/bin/qpdf
+ENV EXIFTOOL_BIN_PATH /usr/bin/exiftool
 
 USER gotenberg
 WORKDIR /home/gotenberg