Runtime support for cel.@block

[TristonianJones]

Introduces runtimes support for cel.@block

CEL block is used by static optimizers to group subexpressions together such that the following is true:

• Each subexpression value may be referenced by its ordinal through the variable: @index<ordinal>
• Subexpressions must be organized such that the expression at @index<N> may only depend on variables represented in @index<N-1>
• Index expressions may not occur within a comprehension loop since each comprehension represents a scope which declares its own variables.

Depends on https://github.com/google/cel-spec/pull/402 before submission