Add support for base64url variant of base-64 encoding/decoding

Add support for base64url variant of base-64 encoding/decoding

Implement the RFC4648 variant of base64 encoding used by URLs.
* doc/lispref/text.texi (base64url-encode-region,
base64url-encode-string): Document new functions.
(base64-decode-region, base64-decode-string): Document new optional
parameter 'base64url' used to use url variant when decoding data.

* src/fns.c (base64url-encode-region, base64url-encode-region): New
functions to manage url variant.
(base64-decode-region, base64-decode-string): Add optional
parameter to indicate use of url-variant.
(base64_encode_region_1, base64_encode_string_1): Internal functions
with extracted code from 'base64_encode_region' and
'base64_encode_string' and optional parameters to manage padding and
url variant.
(base64-encode-region, base64-encode-string) : Use
base64_encode_region_1 and base64_encode_string_1.
(base64-encode-1): Add parameters to manage padding and url variant.
(base64-decode-1): Add parameter to manage url variant.

* test/src/fns-tests.el (fns-tests--with-region): New helper macro to
test region variant of base64 encode / decode functions.
(fns-tests--string-repeat): Helper function used in base64 tests.
(fns-tests-base64-encode-region, fns-tests-base64-encode-string):
Tests for standard base64 function.
(fns-test-base64url-encode-region,
fns-test-base64url-encode-string): Tests for url variant.
(fns-tests-base64-decode-string): Tests for decoding part.