(WIP) Refactor blocking rule for clarity

[RobinL]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Summary

This PR attempts to clarify the BlockingRule class. It is a step towards enabling ExplodingBlockingRules

Things changed:

Clarify name of sql property

🤔 Previously BlockingRule had a sqlproperty which was unused, and a property calledblocking_rulewhich returned the sql. The later was confusing: did it return aBlockingRule` or a sql string?.

Solution: There's now a single property called blocking_rule_sql

Better iteration

-🤔 When UNION ALLing blocking rules, iteration was difficult because a salted blocking rule produces a list of blocking rules as br.salted_blocking_rules, but if it was not salted, you needed to access the blocking rules directly.

So iteration looks like this:

    for br in blocking_rules:
        # Apply our salted rules to resolve skew issues. If no salt was
        # selected to be added, then apply the initial blocking rule.
        if apply_salt:
            salted_blocking_rules = br.salted_blocking_rules
        else:
            salted_blocking_rules = [br.blocking_rule]

Solution: To access all blocking rules and salted blocking rules, the user should now use br.salted_blocking_rule_segments, which is iterable (has 1 element in the case of no salting)

Moving SQL up into BlockingRule class

Previously the SQL statement to be UNION ALLed didn't live with the BlockingRule

      sql = f"""
            select
            {sql_select_expr}
            , '{br.match_key}' as match_key
            {probability}
            from {linker._input_tablename_l} as l
            inner join {linker._input_tablename_r} as r
            on
            ({salted_br})
            {br.and_not_preceding_rules_sql}
            {where_condition}
            """

That was fine when there was only one variant, but to allow [ExplodingBlockingRules](https://github.com/moj-analytical-services/splink/pull/1692) this SQL will change depending on the type of blocking rule

[RobinL]

Closed in favour of #1700 #1701