Add disable promExpr validation feature flag for Cli and operator

[r3code]

Pull from slok#511

This commit would fix issue slok#510

Hi.
promql expr validator in code caused issues because of lack of compatibility with some new promql inspired query engines like MetricsQL.
I think it would be helpful if we add some feature to sloth to allow people to disable promql validation if they want

For example:

i want to create a SLO from this SLI

version: "prometheus/v1"
service: "myservice"
slos:

Latency

• name: "my_metricsql-query"
  objective: 99.5
  description: ""
  sli:
  events:
  error_query: sum(count_over_time((my-metrics{} > 15)) OR on() vector(0) )
  total_query: sum(count_over_time((my-metric{})) OR on() vector(0))
  alerting:
  page_alert:
  disable: true
  ticket_alert:
  disable: true
  but i got error

INFO[0000] SLI plugins loaded plugins=0 svc=storage.FileSLIPlugin version=v0.11.0 window=30d
INFO[0000] SLO period windows loaded svc=alert.WindowsRepo version=v0.11.0 window=30d windows=2
INFO[0000] Generating from Prometheus spec version=v0.11.0 window=30d
error: "generate" command failed: could not generate Prometheus format rules: could not generate prometheus rules: invalid SLO group: Key: 'SLOGroup.SLOs[0].SLI.Events.ErrorQuery' Error:Field validation for 'ErrorQuery' failed on the 'prom_expr' tag
Key: 'SLOGroup.SLOs[0].SLI.Events.TotalQuery' Error:Field validation for 'TotalQuery' failed on the 'prom_expr' tag
My query is actually correct and give me response from VictoriaMetrics in grafana with VictoriaMetrics prometheus data source

My suggestion is adding a capability of disabling promexpr validation with some flag
for example:

sloth generate -i ../my-victoriametrics-queries/test.yml -o /tmp/test.yaml --disable-promExpr-validation