Add DiscoverCredentials method

Nobody has claimed this yet.

In order to support autofill scenarios, we need to provide a method that allows a user to silently discover the existence of credentials.

Some considerations on the design:
- should we allow cross-origin requests (probably specific to credential type)? Is that even meaningful?
- what data should we return?

# Prior art

## WebAuthn

- [[[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors)](https://www.w3.org/TR/webauthn-3/#sctn-discover-from-external-source)-
- [`silentDiscoveryOperation`](https://www.w3.org/TR/webauthn-3/#sctn-op-silent-discovery)

[`DiscoverableCredentialMetadata`](https://www.w3.org/TR/webauthn-3/#discoverablecredentialmetadata) includes the following fields:
```json
{
"type": "public-key",
"id": "credentialID",
"rpId": "example.com"
"userHandle": "abcdefghi1n",
"otherUI": ??
}
```

## Android

I have no idea how Android allows browsers to do credential discovery.
[docs](https://developer.android.com/reference/androidx/credentials/CredentialManager#prepareGetCredential(androidx.credentials.GetCredentialRequest))

## Apple
Browser API:

```swift
platformCredentials(
forRelyingParty relyingParty: String
) async -> [ASAuthorizationWebBrowserPlatformPublicKeyCredential]

struct ASAuthorizationWebBrowserPlatformPublicKeyCredential {
/// A string the person can supply to describe this credential.
let customTitle: String

/// The user name for the account associated with this credential.
let name: String

/// The name of the app that manages this credential, or “iCloud Keychain” if it’s the operating system.
let providerName: String

/// The relying party that issues challenges for this credential.
let relyingParty: String

/// A unique identifier for the user account at the relying party.
let userHandle: Data

/// The identifier the operating system uses for this credential.
let credentialID: Data
}
```
Returns metadata for passkey. Returns credential ID, RP ID, user ID, user name and provider name.

[docs](https://developer.apple.com/documentation/authenticationservices/asauthorizationwebbrowserpublickeycredentialmanager/platformcredentials(forrelyingparty:))

Credential provider side:

```swift
ASCredentialProviderViewController.prepareCredentialList(
for: [ASCredentialServiceIdentifier],
requestParameters: ASPasskeyCredentialRequestParameters
)
```
[docs](https://developer.apple.com/documentation/authenticationservices/ascredentialproviderviewcontroller/preparecredentiallist(for:requestparameters:))

Receives a list of "service identifiers" (URL, domain or app ID), which the credential provider should use to pull up a list of for display.

## Windows

```c
typedef struct _WEBAUTHN_CREDENTIAL_DETAILS {
// Version of this structure, to allow for modifications in the future.
DWORD dwVersion;

// Size of pbCredentialID.
DWORD cbCredentialID;
_Field_size_bytes_(cbCredentialID)
PBYTE pbCredentialID;

// RP Info
PWEBAUTHN_RP_ENTITY_INFORMATION pRpInformation;

// User Info
PWEBAUTHN_USER_ENTITY_INFORMATION pUserInformation;

// Removable or not.
BOOL bRemovable;

//
// The following fields have been added in WEBAUTHN_CREDENTIAL_DETAILS_VERSION_2
//

// Backed Up or not.
BOOL bBackedUp;

//
// The following fields have been added in WEBAUTHN_CREDENTIAL_DETAILS_VERSION_3
//
PCWSTR pwszAuthenticatorName;

// The logo is expected to be in the svg format
DWORD cbAuthenticatorLogo;
_Field_size_bytes_(cbAuthenticatorLogo)
PBYTE pbAuthenticatorLogo;

// ThirdPartyPayment Credential or not.
BOOL bThirdPartyPayment;

//
// The following fields have been added in WEBAUTHN_CREDENTIAL_DETAILS_VERSION_4
//

// Applicable Transports
DWORD dwTransports;

} WEBAUTHN_CREDENTIAL_DETAILS, *PWEBAUTHN_CREDENTIAL_DETAILS;
typedef const WEBAUTHN_CREDENTIAL_DETAILS *PCWEBAUTHN_CREDENTIAL_DETAILS;

typedef struct _WEBAUTHN_GET_CREDENTIALS_OPTIONS {
// Version of this structure, to allow for modifications in the future.
DWORD dwVersion;

// Optional.
LPCWSTR pwszRpId;

// Optional. BrowserInPrivate Mode. Defaulting to FALSE.
BOOL bBrowserInPrivateMode;
} WEBAUTHN_GET_CREDENTIALS_OPTIONS, *PWEBAUTHN_GET_CREDENTIALS_OPTIONS;
typedef const WEBAUTHN_GET_CREDENTIALS_OPTIONS *PCWEBAUTHN_GET_CREDENTIALS_OPTIONS;

HRESULT
WINAPI
WebAuthNGetPlatformCredentialList(
_In_ PCWEBAUTHN_GET_CREDENTIALS_OPTIONS pGetCredentialsOptions,
_Outptr_result_maybenull_ PWEBAUTHN_CREDENTIAL_DETAILS_LIST *ppCredentialDetailsList);
```

[docs](https://github.com/microsoft/webauthn/blob/master/webauthn.h#L1317-L1322)

Allows retrieving credential metadata by RP ID, with flag for browser private mode. Returns credential ID, RP ID, user ID and other information.

No source file or test is named in the issue. Start by reviewing the WebAuthn silent-discovery references and the Android, Apple, and Windows credential APIs cited here, then determine the cross-origin rules, returned metadata, and D-Bus method contract; done means those design questions are resolved in the proposed DiscoverCredentials API.

Written by the indexing model from the issue text.