New trace context spec should define which base64 encoding is expected

[rhcarvalho]

RFC 4648 defines a standard base64 encoding, and an alternate encoding that is typically used in URLs and file names.

These are the alphabets in each:

const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"

The spec should define clearly what set of 64 characters are expected.

The lack of padding is also part of the RFC, so could be referred to in the docs -- https://www.rfc-editor.org/rfc/rfc4648.html#section-3.2