WIP

"Provides" is used because things that need an SSH key can accept either a PassphraseSSHPrivateKeyFileCredentialType (path to a key on disk) or a PassphraseSSHPrivateKeyTextCredentialType (actual key text): both "provide" an SSH key. Specifically, both can build a PassphraseSSHKey object.

The former is a legacy credential type which you can no longer generate from the UI, but which we can't cleanly/easily migrate away from (bin/storage upgrade may not have access to read the key, and the key may not even be present on the host where bin/storage upgrade runs) and which isn't clearly or unambiguously undesirable, although I do think we'd generally be better off without it.

The way this works is generally a little funky / nonstandard.

PassphraseCredential also currently stores a providesType in the database, but this change does not include a migration. My expectation is that it would break every existing SSH key without this migration.

That column probably should not exist, and the Query should implement searches for a particular providesType by enumerating all credential types which provide that type of credential, then doing a WHERE credentialType IN (...) instead. That is, the credentialType always strictly determines the providesType, so the column which stores providesType separately is needless and undesirable.

I think a reasonable pathway forward might look like this:

1. Do this change, but don't change the PROVIDES_TYPE constants, since that will break withProvidesTypes(...) calls on the Query if you don't migrate.
2. Change the Query to resolve withProvidesTypes(...) like this instead:
   • Get all credential types.
   • Find the ones that provide the right PROVIDES_TYPE.
   • Query as credentialType IN (%LS) for those types.
3. Remove the providesType column from the database, as it should no longer have use sites.
4. Maybe change PROVIDES_TYPE to something like withImplements(array $interfaces), then define PassphraseCredentialSSHKeyProviderInterface, etc. Do the lookup in the same way (find matching credential types, query by credential type). This would give us a more standard-looking class tree which was a bit more flexible, I think?