BP_OPENSSL_ACTIVATE_LEGACY_PROVIDER: load legacy ssl provider via env var

[sophiewigmore]

As a follow on from #905, allow users to also load the legacy openssl provider on cflinuxfs4 via a new environment variable, BP_USE_LEGACY_OPENSSL. This can be passed directly when running cf push or via manifest.yml file

add support for new BP_OPENSSL_ACTIVATE_LEGACY_PROVIDER environment variable

• Enabling the new environment variable will load and active the legacy openssl provider on cflinuxfs4
• Removes use_legacy_openssl buildpack.yml setting in favour of the environment variable

[arjun024]

I'm not very fond of BP_USE_LEGACY_OPENSSL because the buildpack is not really instructing the build to use legacy openssl.

• It may be misconstrued as being instructed to use openssl1 or anything < openssl3
  OR
• It may be misconstrued as being instructed to simply use the Openssl3 legacy provider - that's also not true. AFAIU, the config we add instructs openssl3 to load and activate the legacy provider in addition to the openssl3 default provider.^[1]

How about something like BP_OPENSSL_ACTIVATE_LEGACY_PROVIDER? If you think it's too long, trim the _PROVIDER suffix. The verb activate seems to be well known/used with openssl providers. Let's also document it in the CF documentation.

Other than the above, the PR LGTM.

Ref

1. https://wiki.openssl.org/index.php/OpenSSL_3.0#Legacy_Algorithms:~:text=This%20is%20a%20minimal%20config%20file%20example%20to%20load%20and%20activate%20both%20the%20legacy%20and%20the%20default%20provider%20in%20the%20default%20library%20context.

[sophiewigmore]

@arjun024 that's a great point. I didn't think about how misleading that could be. I like BP_OPENSSL_ACTIVATE_LEGACY_PROVIDER, I think it's descriptive (although long). I know we've already released the buildpack.yml change with the name use_legacy_openssl, but do you think we should change that to be openssl_activate_legacy_provider as well?