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/v1/models/event.go

@@ -5,6 +5,8 @@ import (
 	"reflect"
 	"time"
 
+	"github.com/shopspring/decimal"
+
 	"github.com/zitadel/zitadel/internal/eventstore"
 	"github.com/zitadel/zitadel/internal/zerrors"
 )
@@ -20,7 +22,7 @@ var _ eventstore.Event = (*Event)(nil)
 type Event struct {
 	ID               string
 	Seq              uint64
-	Pos              float64
+	Pos              decimal.Decimal
 	CreationDate     time.Time
 	Typ              eventstore.EventType
 	PreviousSequence uint64
@@ -80,7 +82,7 @@ func (e *Event) Sequence() uint64 {
 }
 
 // Position implements [eventstore.Event]
-func (e *Event) Position() float64 {
+func (e *Event) Position() decimal.Decimal {
 	return e.Pos
 }