Expose digest auth checking functions

PUBLISHED_FROM=1bfc6e332f56b68eb6155bb729a97a0d8d5a316c

docs/c-api/http.h/mg_http_is_authorized.md

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+---
+title: "mg_http_is_authorized()"
+decl_name: "mg_http_is_authorized"
+symbol_kind: "func"
+signature: |
+  int mg_http_is_authorized(struct http_message *hm, struct mg_str path,
+                            int is_directory, const char *domain,
+                            const char *passwords_file, int is_global_pass_file);
+---
+
+Checks whether an http request is authorized. `domain` is the authentication
+realm, `passwords_file` is a htdigest file (can be created e.g. with
+`htdigest` utility). If either `domain` or `passwords_file` is NULL, this
+function always returns 1; otherwise checks the authentication in the
+http request and returns 1 only if there is a match; 0 otherwise. 
+