add preset field to SAMLIdPServiceProviderV1 spec

[flyinghermit]

The preset field will be used to process custom configuration for specified service provider. The field can be empty in default state (which means a generic service provider type) but if a value is provided, we expect to be one of the supported one.

kind: saml_idp_service_provider
metadata:
  name: appname
spec:
  preset: gcp (or azure, gitlab etc)
  entity_id: https://samlsp-host

A new package samlsp has been added that defines a preset name for GCP. Subsequent name for other SAML apps, including their custom configuration values (if any) will go into this package.

changelog: The saml_idp_service_provider spec adds a new preset field that can be used to specify pre-built SAML service provider profile.

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[r0mant]

lgtm with a couple of comments

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[public-teleport-github-review-bot]

@flyinghermit See the table below for backport results.

Branch	Result
branch/v15	Failed