[Code] Support Conditional/Atomic HTTP operations to secure BIOS about edk2-redfish-client-deprecated

[nicklela]

This is the design to make sure that BIOS always use latest context

[nicklela]

Enhancement:

• use if-non-match to check resource modification instead of comparing ETag.

Note: OpenBMC has no ETag implementation: https://lore.kernel.org/all/[email protected]/T/

We previously supported etag for static resources, hence the

references to them. They were removed when I moved phosphor-webui to
webpack builds, and it was always on my list to add them back, using
webpacks hashing mechanisms, but I never got a chance.

In practice, they didn't provide that much utility because most browsers:

1. only cache if you have a valid SSL certificate.
2. Have a very limited cache size.
3. Don't support cross site etag caches.

So in practice, the cache didn't provide any benefit in a majority of cases.

In terms of supporting etags on non-static resources, that's quite
complex, given that cache invalidation is difficult, and likely
deserves its own design document.

[nicklela]

Another solution is on BMC side:

• If there is last minute change from end user, BMC won't just clear all context at /Bios/Settings.
• BMC will check and only remove attribute that is consumed by BIOS. Last minute user change will sty in /Bios/Settings and user will know this change is not consumed yet.