Improve DDB bytes encoding

[giograno]

Motivation

In #8206 we introduced a JSON encoder to handle bytes in DDB entries.
However, while the implementation of BytesEncoder fixed runtime encoding errors, did not achieve full parity with AWS since DynamoDB encodes bytes in Base64.
This PR improves the encoder to fix this gap.
This PR will also address #8952.

Changes

• Improve the BytesEncoder encoder by encoding bytes in base64;
• Adding a small unit test;
• Remove a snapshot skip in test_dynamodb_event_source_mapping to verify the correctness of the implementation.

[github-actions]

LocalStack Community integration with Pro

    2 files  ±0      2 suites  ±0   1h 31m 21s ⏱️ +40s
2 922 tests ±0  2 632 ✅ ±0  290 💤 ±0  0 ❌ ±0 
2 924 runs  ±0  2 632 ✅ ±0  292 💤 ±0  0 ❌ ±0 

Results for commit abcd410. ± Comparison against base commit fcd5927.

[viren-nadkarni]

LGTM!