Feature - Infer account executable flag from owners

[Lichtso]

Problem

The executable flag was originally meant to mark finalized programs as read-only. However, with the introduction of the upgradeable loader (loader-v3) this mechanism was circumvented. The workaround in the upgradeable loader is to use a proxy account that gets marked as executable (thus read-only) and which in turn points to the actual program-data account (which is not marked as executable and can be written to).

Hence, the executable flag is useless and gets in the way. It perpetuates the workaround with the proxy account, which in turn complicates transaction account loading, program loading, the program runtime and even CPI for dApp devs. To solve this loader-v4 was designed without the need for a proxy account by ignoring the executable flag. In fact the entire program runtime can ignore the executable flag, instead relying on the owner field and content of program accounts.

Summary of Changes

• Adds Serializer::infer_is_executable() as the VM needs to continue to emulate it (in serialization) to stay ABI compatible
• Adds the feature
• Adds the feature gate logic, which disables all executable flag related errors
• Adds deprecation attributes
• Adjusts tests

[Lichtso]

Closing in favor of #2182.