Moves the Slack notifier's internal types (`Notifier`, `request`,
`attachment`, `slackResponse`) from slack.go into a new types.go.
No behavioral changes.

While moving them, two doc comments were tightened:
- `request`: "slack notification" -> "Slack notification"
- `attachment`: "richly-formatted message block" -> "richly formatted
  message block"

Signed-off-by: Santiago Fernández Núñez <[email protected]>
Co-authored-by: Cursor <[email protected]>

Moves SlackConfig (and the related SlackAction, SlackConfirmationField,
SlackField types plus DefaultSlackConfig) from config/notifiers.go into
a new notify/slack/config.go, following the same pattern used recently
for the mattermost, jira, and incidentio notifiers.

The Slack-specific unmarshal tests are moved alongside them into
notify/slack/config_test.go. config.Receiver now refers to
*slack.SlackConfig, and notify/slack no longer needs to import the
top-level config package.

No behavioral changes.

Signed-off-by: Santiago Fernández Núñez <[email protected]>
Co-authored-by: Cursor <[email protected]>

Previously the Slack notifier hardcoded the chat.update endpoint to
"https://slack.com/api/chat.update" regardless of the configured
api_url. Deployments pointing api_url at a Slack-compatible proxy or
forwarder were therefore forced back onto slack.com for update calls,
which is the wrong host for proxy users.

Move the URL handling into a new internal/apiurl package, and route
chat.update (and any future Slack Web API method) through the same
api_url / api_url_file base as the initial chat.postMessage call.
Standard configurations are unaffected; the generated URL is byte
identical.

The Resolver also reads api_url_file on every call, matching previous
behaviour so that file rotations apply to the next notification without
restarting Alertmanager.

Signed-off-by: Santiago Fernández Núñez <[email protected]>
Co-authored-by: Cursor <[email protected]>