WebAuthn CredentialID field needs to be increased in size (#20530)

WebAuthn CredentialID field needs to be increased in size (#20530)

WebAuthn have updated their specification to set the maximum size of the
CredentialID to 1023 bytes. This is somewhat larger than our current
size and therefore we need to migrate.

The PR changes the struct to add CredentialIDBytes and migrates the CredentialID string
to the bytes field before another migration drops the old CredentialID field. Another migration
renames this field back.

Fix #20457

Signed-off-by: Andrew Thornton <art27@cantab.net>