MEDIUM: ssl: add FIPS signature algorithm check for AWS-LC

MEDIUM: ssl: add FIPS signature algorithm check for AWS-LC

Add ssl_fips_check_sigalgs() which validates the configured signature
algorithm list against the FIPS-approved set: ECDSA on NIST P-curves
with SHA-256/384/512, RSA-PSS (rsae and pss variants) with SHA-256/
384/512, and RSA-PKCS1 with SHA-256/384/512.  SHA-1 based algorithms
and non-FIPS primitives (ed25519, ed448) are rejected.

The check uses the same strchr-based string parsing as
ssl_fips_check_ciphersuites().  A NULL list is silently accepted since
the global defaults were already overwritten with FIPS values at init
time.

The check is called right after SSL_CTX_set1_sigalgs_list() and
SSL_CTX_set1_client_sigalgs_list() in both the bind
(ssl_sock_prepare_ctx) and server (ssl_sock_prepare_srv_ssl_ctx)
configuration paths.