Add Base64URLSafe decoder

[nyanshak]

• Add decoder that can decode base64 strings with '_' and '-' instead of of '+' and '/'.

Addresses #144.

WIP but looking for early feedback:

• should this functionality be merged into the existing base64 decoder or split into a separate Base64URLSafe decoder? There is a lot of shared code and wanted to see if maintainers have preferences towards making separate decoders (like UTF8/UTF16 decoders) or a single Base64 decoder before polishing it up, wiring into all the places (detectorspb, decoders, engine, detectors.proto, etc.

[dustin-decker]

If it's possible to combine the two types of b64 correctly that would be preferred, because there is a decent amount of overhead introduced with each decoder. The UTF8/16 ones are different enough to be separate.

[nyanshak]

Okay @dustin-decker - I combined them. I think there's a possible caveat, which is mostly that combining them adds potentially a little overhead in attempting to decode strings that would not be valid b64-encoded strings.

For example, this is an invalid b64 encoded string: abcdef-ghijkl_mnop+qrs/tuvwxyz. In the separate decoder version, we wouldn't waste time trying to decode this, because once we hit a character outside of the character set, we'd try to decode the substring if it was longer than the threshold, but we wouldn't try to decode the entire string.

If the combined version is good enough, then I think this is ready to go. Otherwise, may need to revert the last commit, then wire up the separate B64URLSafe decoder 🤷‍♀️

[dustin-decker]

Thank you, Brendan!