[Tile-serving] [openstreetmap/osm2pgsql] [Flex] Provide conninfo to Lua script (#1212)

[mmd]

This relates a bit to #1211. Assuming we want to create additional indices, or use database lookups, etc., it would be useful to have access to the database connection information that the C++ code uses, and leverage a library like `luasql.postgres` :

```
luasql = require "luasql.postgres"
envv = assert (luasql.postgres())
con = assert (envv:connect(osm2pgsql.conninfo))

res = con:execute("DROP TABLE people")

res = assert (con:execute[[
        CREATE TABLE people(
                id bigint,
                key text,
                value text
        )
]])
```

Maybe this one liner would be sufficient? There's one caveat: conninfo might contain a password. Not sure, if we can assume Lua scripts to be trustworthy enough here.

```
diff --git a/src/output-flex.cpp b/src/output-flex.cpp
index 4c7afd3e..ad43d349 100644
--- a/src/output-flex.cpp
+++ b/src/output-flex.cpp
@@ -1273,6 +1273,7 @@ void output_flex_t::init_lua(std::string const &filename)
     luaX_add_table_func(lua_state(), "define_table",
                         lua_trampoline_app_define_table);
     luaX_add_table_func(lua_state(), "mark", lua_trampoline_app_mark);
+    luaX_add_table_str(lua_state(), "conninfo",  m_options.database_options.conninfo().c_str());
 
     lua_setglobal(lua_state(), "osm2pgsql");

```



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