squid: Enable dynamic SSL certificate generation

squid: Enable dynamic SSL certificate generation

Maintainer: @neheb / @BKPepe / @zhanhb
Compile tested: ipq806x, generic, netgear_r7800, master
Run tested: ipq806x, generic, netgear_r7800, openwrt-19.07

Description:

Squid now only support HTTPS proxy in TCP tunnel mode (e.g. `ssl_bump splice all`):

    https_port 3128 ssl-bump tls-cert=/etc/squid/squid.pem generate-host-certificates=on
    ssl_bump splice all

In order to operate in SSL Bump mode, we need to compile with `--enable-ssl-crtd` for following configuration:

    https_port 3128 ssl-bump tls-cert=/etc/squid/squid.pem generate-host-certificates=on
    sslcrtd_program /usr/lib/squid/security_file_certgen -s /car/cache/squid/ssl_db -M 4MB
    ssl_bump stare all
    ssl_bump bump all

This PR switch the `SQUID_enable-ssl-crtd` into `default y`, therefore default enable SSL Bump mode.

Signed-off-by: Wong Hoi Sing Edison <hswong3i@pantarei-design.com>