Refactor (and consider moving) base64 and utf8 utils

[larabr]

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We should review if these utils are needed in the library, otherwise move them over to the clients monorepo.

Regardless, the following refactoring should be done:

• Remove the binary string variants encodeUtf8 and decodeUtf8
• Accept/return Uint8Arrays for base64 encoding/decoding
• Remove encodeUtf8Base64 and decodeUtf8Base64 (they can just chain two functions)

decodeBase64 (or base64ToBytes/Array) should return a Uint8Array, and then utf8BytesToString should take a Uint8Array and return a string, so that utf8BytesToString ∘ base64ToBytes == decodeUtf8Base64.

And then if binary strings are needed in some cases, binaryStringToArray / arrayToBinaryString can be used.

[paulmillr]

just use scure-base which is independently audited, minimal, 0-dep base64 lib.