Using `rasn` for Keeta block types

[mamonet]

Options available:

1. rasn - no_std but requires alloc. Uses heap-allocating types internally (bitvec, bytes, etc.). Incompatible with ledger's heapless secure element environment.

2. rasn-compiler custom backend - The compiler supports pluggable backends, so a zero-copy backend is theoretically possible. However, no existing heapless backend exists to build from—would require implementing a full Backend trait from scratch. Effort may exceed benefit for this schema size.

3. Manual implementation (currently proposed Implement keetanetwork-block crate with block and operation types #3) - Direct control over memory layout, zero-copy parsing with &'a [u8] slices, minimal effort compared to building a custom compiler backend.