IMPLEMENT find first method

Implement findFirst method for filename matching. When an enpoint path does not match exactly a filename, a series of fuzzy matches should be tried:

If the path is a directory, try index$whatever files (not only by extension, could also be index_1.json, index_2.json, etc) inside the dir
if there are files with the last part of the path with some extension. For example for /foo/bar there could be bar.json files
if there are multiple matches by file name, parse all of them and check the "match" config in the file config header
if no files are found after all, try to find 404$whatever files (404.json, 404_1.json, etc). They could be either inside the url path directory (for example /foo/bar/404.json, /foo/404.json) or in the root directory

fermin-silva / mockserver