feat: update synthetics implementation according to Kibana 8.16 API changes

[biscout42]

Why

This pr

• adds support of ssl_certificate_authorities, ssl_certificate, ssl_key and ssl_key_passphrase to synthetics monitors (tcp and http). Related to [Feature] Support option ssl.certificate_authorities for elasticstack_kibana_synthetics_monitor #916
• adds plan modifiers for the values, which Kibana sets by default, including alert field of a monitor
• add initial CONTRIBUTING.md, as a first step towards to [Bug] Add contributor docs #826

Warning

ssl_certificate_authorities works only from 8.16.0 due-to elastic/kibana#187952

Note due-to fixes in 8.16 / #964 we can now import passowrd fields from kibana (marked as sensitive in this PR).

TODO

• TF schema - fix [Bug] Inconsistent state for elasticstack_kibana_synthetics_monitor #965
  • bare min test for: tcp, icmp and browser
  • try to setup plan modifier for alert [timeboxed]
• TF schema - fix [Feature] Support option ssl.certificate_authorities for elasticstack_kibana_synthetics_monitor #916
  • schema test for ssl fields
  • acc test for ssl fields, 8.16+ only
• TF schema - test imports after 8.16 fixes
• Update change log