Add a proper API to create memslices from the QUIC code. Current API provides an (allocator, size) constructor. However, this just creates a buffer with random data in it and no way to change it, since memslices are read-only. Current code uses const_cast; this CL creates a proper API and updates all existing call sites to use it. gfe-relnote: n/a (no functional change) PiperOrigin-RevId: 287072774 Change-Id: Ie4dd2eae6db2ec91b087f5d41887ea3948ee411a