encryption: Introduce `Encryptable`

Created by: efritz

Introduces a type to manage lazy/on-demand encrypting/decrypting of values encryptable at-rest in the database.

Several new uses are stacked on top of this definition.

• https://github.com/sourcegraph/sourcegraph/pull/40292
• https://github.com/sourcegraph/sourcegraph/pull/40177
• https://github.com/sourcegraph/sourcegraph/pull/40228
• https://github.com/sourcegraph/sourcegraph/pull/40233
• https://github.com/sourcegraph/sourcegraph/pull/40237

Test plan

Test coverage in linked PRs.