Split `SecretStorage` into two parts (#3267)
* Pull `SecretStorageCallbacks` out of `ICryptoCallbacks`
* Pull the storage part of SecretStorage out to a new class
* Move SecretSharing to a separate class
* Move `ISecretRequest` into `SecretSharing.ts`
* Pull out ISecretStorage interface, and use it
* Mark old `SecretStorage` as deprecated, and rename accesses to it
* Move a `SecretStorage` unit test into its own file
* Use new `SecretStorage` in a couple of places
* add some more unit tests
* Fix test file name
... to match the unit under test
* even more tests
* Add a load of comments
* Rename classes
* Fix some broken tsdoc links
* fix broken test
* Fix compaints about superlinear regex
* just one more test
