Support Age encryption format

[hiddeco]

With SOPS v3.7.0 merged, we are now able to add support for the Age encryption format.

Rough sketch of what needs to be done:

• Refactor the KustomizeDecryptor so that only .asc key/value pairs from the referred secret are written to disk.
• Collect .age key/value pairs (in bytes) to be used for decryption using Age.
• Adapt our own sops/keyservice server so that it is capable of encrypting and decrypting the Age encryption format with the keys we collected from the secret. Inspiration for this can be drawn from from https://github.com/mozilla/sops/blob/66043e71a81787d6513bc2e5505a29aac67dc6f1/keyservice/server.go#L294-L301. As https://github.com/mozilla/sops/blob/master/age/keysource.go can simply be loaded with bytes, we likely won't have to do much more than constructing the keysource with the right values, and then use it to decrypt.