Provide core-utils crate containing base64 helpers

[Fraser999]

We want to use a URL-safe alphabet for all cases where we base64-encode values.

To that end, we should provide a new crate named astria-core-utils which will provide opinionated helpers to support base64 encoding and decoding, i.e. the helpers will exclusively use a URL-safe alphabet and include padding internally and offer no other options to the caller.

This work should also include replacing all existing instances of base64-encoding/-decoding with the new helpers, and should remove the base64 crate from all manifests other than the one for the new core-utils crate.

┆Issue Number: ENG-913

[joroshiba]

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