rename the TLS_CREDENTIAL_SERVER_CERTIFICATE enum to more appropriate name to indicate either Server or client public certificate in the TLS credential subsystem

[VivekUppunda]

Currently the enum values defined for tls_credential_type mention TLS_CREDENTIAL_SERVER_CERTIFICATE for public server certificate.

Either an enum value for client certificate be added or rather make the current enum value TLS_CREDENTIAL_SERVER_CERTIFICATE be renamed to a more appropriate name to indicate public certificates in general.

[rlubos]

I agree TLS_CREDENTIAL_SERVER_CERTIFICATE name is unfortunate, it implies server however in reality it doesn't really make difference if the public ceriticate is registered for a client or server device. Even in current codebase we use this credential type to in fact store client certificate (like for example in LwM2M, cc @SeppoTakalo):

zephyr/subsys/net/lib/lwm2m/lwm2m_engine.c

Line 1006 in 2ca0f5f

ret = load_tls_type(ctx, 3, TLS_CREDENTIAL_SERVER_CERTIFICATE);

I propose we deprecate this name, and rename it to TLS_CREDENTIAL_PUBLIC_CERTIFICATE