Expose digest auth checking functions

PUBLISHED_FROM=1bfc6e332f56b68eb6155bb729a97a0d8d5a316c

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

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+---
+title: "mg_http_send_digest_auth_request()"
+decl_name: "mg_http_send_digest_auth_request"
+symbol_kind: "func"
+signature: |
+  void mg_http_send_digest_auth_request(struct mg_connection *c,
+                                        const char *domain);
+---
+
+Sends 401 Unauthorized response. 
+