fix(conformance): load the metadata the tools ship, echo only requested extensions

[Bccorb]

Closes #172.

The FIDO2 conformance test tools were run against a local instance for the first
time. The first run reported 15 failures. This fixes the three defects behind
them, and a run now reports zero failures.

All three are confined to FIDO_CONFORMANCE_MODE, which is refused under a
production NODE_ENV, so no deployed behaviour changes.

Metadata statements were never loaded

The tools' "DOWNLOAD SERVER METADATA" archive unzips to a nested
metadataStatements/ directory, and the loader only read JSON files at the top
level of FIDO_CONFORMANCE_METADATA_DIR. It silently found none.

With requireKnownAuthenticator set, the metadata service runs in strict mode,
so every conformance authenticator was refused as unlisted and every
registration failed. That cascaded into the tests which depend on a registration
having already succeeded, so the failures pointed away from the actual cause.
Confirmed directly against the library before and after:

before: REFUSE packed Secp256R1 -> No metadata statement found for aaguid "326adcf0-..."
after:  OK     packed Secp256R1 -> Virtual Secp256R1 FIDO2 Conformance Testing CTAP2 Authenticator

The directory is now read recursively, so the archive can be dropped in unedited
as the documentation already promised.

Vendor attestation roots blocked their own tests

The tools sign their Apple, Android Key and SafetyNet statements with their own
test roots, so validating those against the real vendor roots could never
succeed. Those preset roots are now cleared in conformance mode, which lets the
roots carried in the metadata statement apply instead. Android Key attestation
verifies for the first time as a result.

Registration options advertised an extension nobody asked for

generateRegistrationOptions always appends its own credProps, and the tools
compare the echoed extensions against the requested set for exact equality:

AssertionError: Response.extensions MUST be set to the requested extensions!
Expected "{"example.extension.bool":true,"credProps":true}"
to equal "{"example.extension.bool":true}"

The requested set is now echoed verbatim over the library's answer. The
authentication options path was already correct, since the library passes
extensions through there unchanged.

Verification

Three conformance runs, 347 captured requests in the last one. Every options
call answered ok, no ceremony was lost, and every remaining refusal is a
negative test the tools intend to fail: corrupted attestation internals, bad
origins, unsupported formats.

Attestation formats verified end to end during a run: packed, tpm, fido-u2f,
android-key and none. Android SafetyNet is never exercised by the tools even
with every test group selected, which is worth confirming before closing #210.

Full suite green: 1219 passed, 1 skipped. Lint, typecheck, format and coverage
thresholds all clean.