Allow to pass a block to graphql mutation helper

What does this MR do?

Adds a syntactic sugar for the graphql_mutation method which allows passing a block as a GraphQL query.

Example before

graphql_mutation(:update_alert_status, variables.merge(input),
                 <<~QL
                   clientMutationId
                   errors
                   alert {
                     iid
                     status
                   }
                 QL
)

Example after

graphql_mutation(:update_alert_status, variables.merge(input)) do
  <<~QL
     clientMutationId
     errors
     alert {
       iid
       status
     }
  QL
end

Exception output example

Failure/Error: raise ArgumentError, 'Please pass either `fields` parameter or a block to `#graphql_mutation`, but not both.' if fields.present? && block_given?

ArgumentError:
  Please pass either `fields` parameter or a block to `#graphql_mutation`, but not both.
# ./spec/support/helpers/graphql_helpers.rb:81:in `graphql_mutation'

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