Add environment information in Alerts related GraphQL schema

What does this MR do?

A second iteration for #232492 (closed), this MR aims at making environment information available via graphQL.

Note that the feature is behind the enable_environment_path_in_alert_details feature flag, this MR addresses the following scenarios:

A) feature flag disabled	B) feature flag enabled	C) feature flag enabled	D) feature flag enabled
environment is not queried from graphQL	environment is queried from graphQL	environment is queried from graphQL	environment is queried from graphQL but it's null for some reason
path will always be null	path is returned and it's a string	path is null for some reason
The page looks exactly as it is right now	The page has the environment displayed (in the table, environment is above details)	The page looks as in B but the name of the environment is not a link	The page will not display the environment in the summary but it will have an empty row in the table

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team