[C++] Fix wrong Base64 paddings

[BewareMyPower]

Motivation

This bug was introduced from #11492, which adds padding characters but it will still add 4 =s even if the length can be divided by 4. Following tests are broken currently:

• ProtobufNativeSchemaTest.testAutoCreateSchema
• ProtobufNativeSchemaTest.testEndToEnd
• ProtobufNativeSchemaTest.testSchemaJson
• ProtobufNativeSchemaTest.testSchemaIncompatibility

Modifications

Only add at most 2 padding characters in Base64 encoding. If the Base64 encoded string's length is 4N+1, there must be something wrong with the encoding, so throw an exception in this case.

Verifying this change

Currently CI of C++ client is broken, we only need to confirm with the workflow output to ensure this fix works.

[BewareMyPower]

Following output is from https://github.com/apache/pulsar/pull/11578/checks?check_run_id=3259005013.

FAILED TESTS (12/248):
      47 ms: ./main AuthPluginTest.testTlsDetectHttps (try #1)
      54 ms: ./main AuthPluginTest.testTlsDetectHttps (try #2)
      31 ms: ./main BasicEndToEndTest.testV2TopicHttp (try #1)
      54 ms: ./main BasicEndToEndTest.testV2TopicHttp (try #2)
      23 ms: ./main BasicEndToEndTest.testHandlerReconnectionLogic (try #1)
      20 ms: ./main BasicEndToEndTest.testHandlerReconnectionLogic (try #2)
      24 ms: ./main AuthPluginToken.testTokenWithHttpUrl (try #1)
      22 ms: ./main AuthPluginToken.testTokenWithHttpUrl (try #2)
    1535 ms: ./main ClientTest.testConnectTimeout (try #1)
    1517 ms: ./main ClientTest.testConnectTimeout (try #2)
      84 ms: ./main ClientDeduplicationTest.testProducerDeduplication (try #1)
      61 ms: ./main ClientDeduplicationTest.testProducerDeduplication (try #2)

We can see ProtobufNativeSchemaTest didn't fail after the change.