Add semver CI check (#4279)

This checks changed files against API surface changes against what the
prdoc says.

It will error if the detected semver change is greater than the one
listed in the prdoc. It will also error if any crates were touched but
not mentioned in the prdoc.

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>