Port pluggable cryptography and re-wire current cryptography as default

[akafredperry]

Is your feature request related to a problem? Please describe.

The current cyrptography is hard coded in SelfKeyCommands, SharedKeyCommands, PublicKeyCommands and both of AtClientImpl's notification handlers. An application cannot supply a different scheme.

NOTE: Metadata.AppMetadata — the field that would carry the cryptography scheme / provider — already exists on trunk but nothing in the repo reads or writes it.

Describe the solution you'd like

A new interface CryptographyProvider which defines 2 methods encrypt/decrypt. This is mirrors CryptoProvider in Dart.
The existing cryptography is moved into an implementation LegacyCryptographyProvider. This mirrors LegacyCryptoProvider in Dart.
RuntimeCryptographyProvider is a implementation with a map of registered implementations keyed by provider ids. It's encrypt/decrypt implementation will delegate to a registered implementation based on the key Metadata.AppMetadata. Where this is not set it will default.
AtClientImpl and the static command method it invokes to execute at server commands will be modified to invoke a CryptographyProvider.
AtClients builder will be modified to pass a RuntimeCryptographyProvider, with LegacyCryptoProvider as the default, to the AtClientImpl it constructs.

Dart mapping

Dart	Java
CryptoProvider (crypto.dart)	CryptographyProvider (api)
CryptoProvider.id	dropped — the id is the registry key, not provider behaviour
CryptoContext {atClient, atKeysIo} (crypto.dart)	dropped — (AtCommandExecutor, AtCommandExecutorContext)
CryptoConfig {defaultProviderId, providers, lookup} (crypto.dart)	dropped — RuntimeCryptographyProvider's builder
CryptoConfig.lookup(id) linear scan over a List	Map<CryptographyProviderId, CryptographyProvider>
legacyCryptoProviderId = 'legacy' (crypto.dart)	CryptographyProviderId.LEGACY — same wire literal
CryptoRuntime (crypto_runtime.dart)	RuntimeCryptographyProvider (impl.crypto)
encryptForPut / encryptForNotification	CryptographyProvider.encrypt
decryptForGet / decryptForNotification / decryptForSyncConflict	CryptographyProvider.decrypt
LegacyCryptoProvider (legacy/legacy_crypto_provider.dart)	LegacyCryptographyProvider (impl.crypto)
LegacyEncryption.build / LegacyDecryption.build runtime key sniffing	private methods per Keys.AtKey subtype
SelfKeyEncryption / SelfKeyDecryption	encryptSelfKey / decryptSelfKey
SharedKeyEncryption / SharedByMeDecryption / SharedWithMeDecryption	encryptSharedKey / decryptSharedKey
AbstractAtKeyEncryption (shared-key management)	the shared-key helpers inside LegacyCryptographyProvider
AppMetadata.providerId (String, at_commons)	Metadata.AppMetadata.providerId typed as CryptographyProviderId

Describe alternatives you've considered

like for like mapping

Additional context

No response