Add alternative to shasum that doesn't require perl dep such as using sha256sum

### Description

shasum is used in:
https://github.com/Blockstream/green_android/blob/c644e2b751b5a543b69bca4d3fcb27b6922054d8/README.md#L67
https://github.com/Blockstream/green_android/blob/cfa2ecff9ea53dc58ccfb83614961f84f4b4add2/app/fetch_gdk_binaries.sh#L21
https://github.com/Blockstream/green_android/blob/cfa2ecff9ea53dc58ccfb83614961f84f4b4add2/app/fetch_gdk_binaries.sh#L38

in alpine the shasum come from `perl-utils` package, in debian from `perl` package.
both distros have sha256sum as part of `coreutils` package that is installed by default (in debian:sid-slim / alpine:edge) thus better to use it instead if available and not require the `shasum` command that depend on perl installation.

### Expected behaviour

it's possible to run the `app/fetch_gdk_binaries.sh` script without installing perl, sha256sum usage is used instead if installed, and it's also described in README file.