Reuse base64 encoding/decoding methods from vodozemac

[poljar]

We have those same base64 decoding/encoding methods defined in multiple places and it's annoying to keep them in sync. Let's define them once and reuse them instead.

Closes: #2531.

@BillCarsonFr if you have a unit test for the bindings feel free to push a commit or pass the commit to me over Matrix.

[codecov]

Codecov Report

Patch coverage: 80.00% and project coverage change: -0.01% ⚠️

Comparison is base (eb865f4) 77.45% compared to head (cc1e772) 77.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2532      +/-   ##
==========================================
- Coverage   77.45%   77.45%   -0.01%     
==========================================
  Files         180      180              
  Lines       19361    19357       -4     
==========================================
- Hits        14996    14992       -4     
  Misses       4365     4365              

Files Changed	Coverage Δ
...trix-sdk-crypto/src/file_encryption/attachments.rs	79.72% <ø> (ø)
...es/matrix-sdk-crypto/src/olm/signing/pk_signing.rs	95.79% <ø> (ø)
crates/matrix-sdk-crypto/src/store/mod.rs	82.31% <0.00%> (ø)
crates/matrix-sdk-crypto/src/utilities.rs	96.96% <ø> (-0.33%)	⬇️
...atrix-sdk-crypto/src/file_encryption/key_export.rs	97.46% <100.00%> (ø)
crates/matrix-sdk-crypto/src/olm/account.rs	85.67% <100.00%> (ø)

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