feat: add support for base64url alphabet

[grjte]

Description

This completes the process of making the library configurable for padded/unpadded inputs and the commonly required base64 alphabets (standard and base64url).

Problem*

Resolves #15 and also addresses asserts for input/output lengths during decoding as mentioned in #19.

Summary*

• fix issue with loop bound for decoding padded inputs and add length asserts
• add configurable support for base64url
• add new tests for base64url input
• update docs with new configuration and performance numbers

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[grjte]

@saleel I have questions about 2 points:

1. I defined the configurations with the prefix BASE64_, e.g. BASE64_ENCODER_STANDARD or BASE64_ENCODER_STANDARD_NO_PAD. I did it this way because the previous function names were prefixed with base64_, but these seem really long to me, and maybe it would be better to omit the prefix and just keep base64 in the lib, e.g. noir_base64::ENCODER_STANDARD. Wdyt?
2. Let me know if you want the performance benchmarks done differently. I'm not sure if you guys have a particular system/pattern you follow. If so, I'm happy to update

[saleel]

Looks good 🚀

1. Yes, that make sense. Users can alias when importing if needed.
2. I don't think there are any specific patterns now, but we have been discussing about it. What you have now is perfect - one thing we can mention is that the benchmarks are for barretenberg backend (as this is a Noir library and Noir is backend agnostic). Its arguable if backend specific things should even be there in this repo, but we can keep it here for now and move it elsewhere once we have a different pattern / multiple backends.

[grjte]

Thanks @saleel! I've updated with those 2 changes