SimpleStreamBuf data access

[JBKacerovsky]

Issue #, if available:

Description of changes:

Added getters for private members m_buffer and m_bufferSize to SimpleStreamBuf. In my project I needed to pass objects downloaded from s3 to another function as char * this minor change made accessing the data a bit more convenient than using SimpleStreamBuf::str()

In benchmarks it is also marginally faster, but the difference is negligible.

Check all that applies:

• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.) Very minor change. Only exposes access to existing member variables without functional changes.
• [ ] Checked if this PR is a breaking (APIs have been changed) change.
• [ ] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [ ] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [X] Linux
• [ ] Windows
• [ ] Android
• [ ] MacOS
• [ ] IOS
• [ ] Other Platforms

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