Add .github/workflows/getSize.sh to extract sizes of sections from the
objfile
build-firmware uses getSize.sh to output the section sizes.
get-base-ref-size job added, which builds the base branch of the PR and
outputs the section sizes. Caches are used to avoid unnecessary builds
when the base branch hasn't been updated.
leave-build-size-comment job added, which creates or updates a comment
on the PR with the build size information from other jobs.
