misc: Add strict typing to attachment manager

[PlasmaPower]

Feel free to reject this PR if the new abstraction is too complicated or otherwise not your preference, but it fixes a couple of potential pitfalls and simplifies the external API:

• You no longer have to manage the eAttachmentType and figure out which enum values correspond to which types. This also makes it easier to add new attachment types in the future, and provides flexibility to use this in other non-DRM contexts or with external attachment types.
• You no longer have to cast returned attachments to the right type, as the attachment manager now does the cast for you. This also avoids the potential pitfall of casting to the wrong attachment type.
• Previously, we called removeByType before add because it was possible to have multiple attachments of the same type by accident, but attachments are now stored in a map instead of a vector to guarantee uniqueness.

[vaxerski]

generally fine, depends on RTTI but we already do in many places

[vaxerski]

thanks!