RLS: Add support for parameterized queries

[joshua-spacetime]

Support the @sender parameter as defined by the RLS proposal.

/// A user only has access to their row
#[spacetimedb::client_visibility_filter]
const USER_FILTER: Filter = Filter::Sql("
    SELECT * FROM user WHERE identity = @sender
");

[bfops]

P1 - Need to support @sender

[joshua-spacetime]

This will be implemented initially as syntactic sugar meaning that we will resolve the @sender parameter at parse time.

Implications:

• This changes the semantics of sql string comparison
  • Two clients can submit byte-wise equivalent, but not semantically equivalent sql queries
  • Will have to review everywhere that we compare or hash sql strings
  • One place that I know of is when a new subscription is made

Required tests:

• @sender query
• @sender subscription
• @sender subscriptions (byte-wise equal) from two different clients

[joshua-spacetime]

Closed by #2483