Common way to generate and read versions for v3 and v4

Follow up from https://github.com/Azure/azure-functions-host/pull/7346 .

Currently we have v4.x which outputs version.txt https://github.com/Azure/azure-functions-host/blob/v4.x/build/common.props#L25-L31 and dev which builds version number directly from common.props. Need to figure out a way to add more flexibility that can be reused by both v3 and v4.

cc: @brettsam @fabiocav

Read build/common.props on the v4.x branch and the follow-up PR 7346, then compare how the dev branch derives its version from common.props. Done means there is a flexible approach to generating and reading versions that can be reused by both v3 and v4.

Written by the indexing model from the issue text.