httpc: add proxy server options for curl

Added support for the following curl options:
* CURLOPT_PROXY
* CURLOPT_PROXYPORT
* CURLOPT_PROXYUSERPWD
* CURLOPT_NOPROXY

@TarantoolBot document
Title: httpc: proxy server options

Use 'proxy' option to specify the proxy server host or IP-address
(optionally may be prefixed with a scheme - e.g. http:// or https://).
'proxy_port' and 'proxy_user_pwd' options may be used to specify the
proxy port (443 for https proxy and 1080 for others by default) and
user credentials (format: [user name]:[password]) respectively.

If 'proxy' option is not set a value from the corresponding
environment variable will be used. Environment variable names are:
'http_proxy', 'https_proxy', 'ftp_proxy' etc. 'all_proxy' variable
is used if no protocol specific proxy was set.

Setting 'proxy' option to an empty string will explicitly disable the
use of a proxy, even if there is an environment variable set for it.

Set 'no_proxy' option to specify a comma separated list of hosts that
do not require a proxy to get reached, even if one is specified by
'proxy' option (or the corresponding environment variable). The only
wildcard available is a single * character, which matches all hosts,
and effectively disables the proxy. 'no_proxy' environment variable
will be used if this option is not set.

Setting 'no_proxy' option to an empty string will explicitly enable
the proxy for all host names, even if there is an environment variable
set for it.

See:
https://curl.haxx.se/libcurl/c/CURLOPT_PROXY.html
https://curl.haxx.se/libcurl/c/CURLOPT_PROXYPORT.html
https://curl.haxx.se/libcurl/c/CURLOPT_USERPWD.html
https://curl.haxx.se/libcurl/c/CURLOPT_NOPROXY.html