[Rust] add the process step for the creation of a custom binary view

[rbran]

This allows custom implementations of CustomBinaryView to call self.* function related to BinaryView.

Currently calling something like add_segment inside the init function will cause a panic because the context.state had being initialized with CustomViewContextState::Initialized yet, eg minidump does it:

binaryninja-api/view/minidump/src/view.rs

Lines 261 to 265 in e1efc28

	self.add_segment(
	Segment::builder(segment.mapped_addr_range.clone())
	.parent_backing(segment.rva_range.clone())
	.is_auto(true)
	.flags(segment_flags),

)