Validate TRUSTED_PROXIES on server startup

[micahlee]

Currently, if TRUSTED_PROXIES contains an invalid (non-CIDR) value, the first attempt to handle a request will fail with:

IPAddr.new('invalid-value')
Traceback (most recent call last):
        7: from /usr/local/bin/irb:23:in `<main>'
        6: from /usr/local/bin/irb:23:in `load'
        5: from /usr/local/lib/ruby/gems/2.7.0/gems/irb-1.2.3/exe/irb:11:in `<top (required)>'
        4: from (irb):2
        3: from (irb):2:in `new'
        2: from /usr/local/lib/ruby/2.7.0/ipaddr.rb:557:in `initialize'
        1: from /usr/local/lib/ruby/2.7.0/ipaddr.rb:598:in `rescue in initialize'
IPAddr::InvalidAddressError (invalid address: invalid-value)

Conjur should validate that TRUSTED_PROXIES contains valid CIDR values at server startup, to give immediate feedback, rather than wait for the first request.