fix(eventstore): use decimal for position (#9881)

Float64 which was used for the event.Position field is [not precise in
go and gets rounded](https://github.com/golang/go/issues/47300). This
can lead to unprecies position tracking of events and therefore
projections especially on cockcoachdb as the position used there is a
big number.

example of a unprecies position:
exact: 1725257931223002628
float64: 1725257931223002624.000000

The float64 was replaced by
[github.com/jackc/pgx-shopspring-decimal](https://github.com/jackc/pgx-shopspring-decimal).

Rename `latestSequence`-queries to `latestPosition`

closes https://github.com/zitadel/zitadel/issues/8863

internal/eventstore/event_base.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/shopspring/decimal"
 	"github.com/zitadel/logging"
 
 	"github.com/zitadel/zitadel/internal/api/authz"
@@ -25,7 +26,7 @@ type BaseEvent struct {
 	Agg *Aggregate
 
 	Seq                           uint64
-	Pos                           float64
+	Pos                           decimal.Decimal
 	Creation                      time.Time
 	previousAggregateSequence     uint64
 	previousAggregateTypeSequence uint64
@@ -38,7 +39,7 @@ type BaseEvent struct {
 }
 
 // Position implements Event.
-func (e *BaseEvent) Position() float64 {
+func (e *BaseEvent) Position() decimal.Decimal {
 	return e.Pos
 }