[core] Move srt_rejectreason_str implementation to srt_c_api

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[maxsharabayko]

This PR also fixes the missing extern "C" in common.cpp for API function srt_rejectreason_str() and API array srt_rejectreason_msg.
I am not sure though why the srt_rejectreason_msg is present in the API (added in v1.3.4 in PR #720).
It is quite unsafe because the size is not known from the above. 🤔

[quink-black]

This PR also fixes the missing extern "C" in common.cpp for API function srt_rejectreason_str() and API array srt_rejectreason_msg. I am not sure though why the srt_rejectreason_msg is present in the API (added in v1.3.4 in PR #720). It is quite unsafe because the size is not known from the above. 🤔

srt_rejectreason_msg shouldn't be present in the API.

• But once it's there, users may pass enum SRT_REJECT_REASON directly as index to the array. So it cannot be removed directly.
• Add array size to the API may misleading users to use the array directly.

I think it should be marked as deprecated.