Have Svc/CmdDispatcher use hash to store/lookup opcodes

[timcanham]

F´ Version	v3.5.0
Affected Component	/Svc/CmdDispatcher

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Feature Description

Have CmdDispatcher use a hash table to store command opcode registrations and to look up opcodes for dispatching.

Rationale

The current implementation uses a linear search to find a registration slot:

https://github.com/nasa/fprime/blob/devel/Svc/CmdDispatcher/CommandDispatcherImpl.cpp#L30

Likewise, when an opcode is received, the same linear search is made to dispatch it:

https://github.com/nasa/fprime/blob/devel/Svc/CmdDispatcher/CommandDispatcherImpl.cpp#L30

This approach won't scale well for large numbers of commands, so a hash table (like the one on Svc/TlmChan would make the lookups faster.