consolidate _handshakeBuffer and _internalBuffer inside SslStream

[wfurt]

related to #49346
related to #49000

both buffers are currently rented from ArrayBuufer. Aside from code duplication we need to copy leftovers when we transition from handshake to read, read to handshake during TLS 1.3 processing or during renegotiation/late cert retrieval.

using single buffer would simplify internal logic and reduce code.

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Issue Details

---

related to #49346
related to #49000

both buffers are currently rented from ArrayBuufer. Aside from code duplication we need to copy leftovers when we transition from handshake to read, read to handshake during TLS 1.3 processing or during renegotiation/late cert retrieval.

using single buffer would simplify internal logic and reduce code.

Author:	wfurt
Assignees:	-
Labels:	area-System.Net.Security, enhancement
Milestone:	-