Add option to specify external directories for Tesseract

[umireon]

The FlatHub maintainer required us to obtain dependencies from their upstream. I have made changes to use the external tarball that is downloaded from the upstream. I use the build scripts on https://github.com/occ-ai/obs-ocr-deps and the script is not copied into this repository. The pre-downloaded tarballs can be used with CMake command as the following:

cmake . -B build_x86_64 -G Ninja -DQT_VERSION=6 \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo \
  -DENABLE_FRONTEND_API=ON \
  -DENABLE_QT=ON \
  -DBUILD_TESSERACT_FROM_SOURCE=ON \
  -DTESSERACT_SOURCE_URL="/path/to/tesseract-5.3.3.tar.gz" \
  -DTESSERACT_SOURCE_HASH=SHA256=dc4329f85f41191b2d813b71b528ba6047745813474e583ccce8795ff2ff5681 \
  -DLEPTONICA_SOURCE_URL="/path/to/leptonica-1.84.1.tar.gz" \
  -DLEPTONICA_SOURCE_HASH=SHA256=ecd7a868403b3963c4e33623595d77f2c87667e2cfdd9b370f87729192061bef

Refer to flathub/flathub#4907 (comment)

The changes in occ-ai/obs-ocr-deps#1 should be merged before this PR is merged.

[umireon]

@royshil

[royshil]

ok this checks out. lets make sure its only acativated on linux

[umireon]

@royshil Please approve occ-ai/obs-ocr-deps#1 first.

[umireon]

I will need to change the ref to occ-ai/obs-ocr-deps on main.

[royshil]

go