Raw Public Keys: Configure and advertise available client cert types

This CL adds a new SSL_CREDENTIAL type for Raw Public Keys. This type of
credential must have a public key and (either a private key or private
key method) configured in order to be considered complete.

This CL implements advertising the caller's available cert types (as a
client) in the client_certificate_type extension in ClientHello. By
default, this is automatically populated based on the configured
credential list.

The caller, as a client, may also configure the credential list late, or
may want to deviate from the the available client_certificate_types
computed by default. For such cases, this CL also adds an API function
to explicitly configure available client cert types offered in the
ClientHello.

Advertising the server_certificate_type extension in ServerHello, which
depends on the configured credential list as well as other decisions, is
implemented later.

Bug: 467663225
Change-Id: I36da8c1a135cfb423155cf9c7b8d8dc06a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/89829
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Lily Chen <chlily@google.com>