math/rand/v2: add Read method to ChaCha8 source

[mateusz834]

Performance on amd64:

          │     B/s      │

Read/1-12 118.7Mi ± 1%
Read/2-12 239.6Mi ± 0%
Read/3-12 353.1Mi ± 1%
Read/4-12 482.8Mi ± 1%
Read/8-12 949.9Mi ± 1%
Read/16-12 1.313Gi ± 1%
Read/32-12 1.795Gi ± 0%
Read/64-12 2.083Gi ± 1%
Read/128-12 2.249Gi ± 2%
Read/256-12 2.314Gi ± 1%
Read/512-12 2.355Gi ± 1%
Read/1024-12 2.384Gi ± 0%
Read/2048-12 2.373Gi ± 3%
Read/16384-12 2.415Gi ± 0%
geomean 1.090Gi

Fixes #67059

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