Add signature_algorithms field to mbedtls_ssl_config

[yuhaoth]

Supported signature algorithms structures are different between TLS1.2 and TLS1.3. Origin sig_hashes represents hash field defined in RFC5246, but TLS1.3 does not define the field. TLS1.3 define an enum to represent signaure/hash pairs, and the enum is not compatible with TLS1.2.

So, a new field is needed for supported signature algorthms of TLS1.3.

References:

• RFC8446 Section 4.2.3
• RFC5246 Section 7.4.1.4.1