Add webhook message and title fields

[skatsaounis]

Fixes #817

[CLAassistant]

All committers have signed the CLA.

[github-actions]

In order to lower resource usage and have a faster runtime, PRs will not run Cloud tests automatically. To do so, a Grafana Labs employee must promote the Drone build.

For maintainers, it's better to run only the Cloud tests you need, rather than all of them. You can do so by setting the following parameter when promoting:

TESTARGS='-run=<testname>'

[skatsaounis]

Tests are passing with Grafana OSS 9.3.11 inside a Docker container:

docker run --rm -p 3000:3000 grafana/grafana:9.3.11
GRAFANA_URL=http://localhost:3000 \
GRAFANA_AUTH=admin:admin \
GRAFANA_ORG_ID=1 \
TF_ACC_OSS=true \
TF_ACC=1 \
GRAFANA_VERSION="9.3.11" \
go test ./internal/resources/grafana/resource_alerting_contact_point_test.go -v
=== RUN   TestAccContactPoint_basic
=== PAUSE TestAccContactPoint_basic
=== RUN   TestAccContactPoint_compound
--- PASS: TestAccContactPoint_compound (2.64s)
=== RUN   TestAccContactPoint_notifiers
=== PAUSE TestAccContactPoint_notifiers
=== CONT  TestAccContactPoint_basic
=== CONT  TestAccContactPoint_notifiers
--- PASS: TestAccContactPoint_basic (1.87s)
--- PASS: TestAccContactPoint_notifiers (2.94s)
PASS
ok  	command-line-arguments	5.898s

[inkel]

LGTM. @alexweav would you like to take a look?

[alexweav]

LGTM