feat(middleware): Add a MaxBodyBytes configuration option

[vejipe]

Add a MaxBodyBytes configuration option in order to limit cached response body size in Upstream/Revalidate requests.

[vejipe]

General questions:

• Do we want to rename the config option max_cachable_body_bytes to make it clear that its value only refers to bodies stored in cache, not a body going through Souin uncached?
• Corollary: do we want another max_body_bytes that refers to any body coming from upstream. Because currently, the upstream body is first completely copied in RAM and that will because an issue if the content is very large (e.g., several GB video file). Or maybe CustomWriter can be refactored to stream the upstream content and then max_body_bytes would become the maximum body chunk kept in RAM before streaming is engaged?

[darkweak]

Second point will be handled in a future PR.