fix: attachment encoding fallback base64 + base64url by default

Description:

Adding base64 and base64url fallback in didcomm attachment extract fn's. earlier we were using Buffer which was able to handle cases where we get base64url and we should be getting base64.

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[ ] My PR follows the contribution guidelines of this project
[ ] My PR is free of third-party dependencies that don't comply with the Allowlist
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hyperledger / identus-edge-agent-sdk-ts

fix: attachment encoding fallback base64 + base64url by default #239

Description:

Checklist:

JUnit