[1.0.1 -> main] Make signed_block_ptr const

[heifner]

Change signed_block_ptr to be std::shared_ptr<const signed_block> instead of std::signed_block>.
Requires allowing fc::unpack of std::shared_ptr<T> to cast away const so it can be overwritten.
Didn't turn out too bad.

Merges release/1.0 into main including #744

[ericpassmore]

Note:start
category: Chores
component: Internal
summary: Utilize compiler to ensure blocks are treated as immutable after construction within implementation.
Note:end