feat: adding custom/default user agent support for http monitors

[akashmannil]

Custom and Default User agent support for monitoring.

Checkmate/X.X (uptime monitor; your-instance) - Placeholder added keeping in mind of the local checkmate instance. Please do let me know if changes needed

Implements #3260

Please ensure all items are checked off before requesting a review. "Checked off" means you need to add an "x" character between brackets so they turn into checkmarks.

• (Do not skip this or your PR will be closed) I deployed the application locally.
• (Do not skip this or your PR will be closed) I have performed a self-review and testing of my code.
• I have included the issue # in the PR.
• I have added i18n support to visible strings (instead of <div>Add</div>, use):

const { t } = useTranslation();
<div>{t('add')}</div>

• I have not included any files that are not related to my pull request, including package-lock and package-json if dependencies have not changed
• I didn't use any hardcoded values (otherwise it will not scale, and will make it difficult to maintain consistency across the application).
• I made sure font sizes, color choices etc are all referenced from the theme. I don't have any hardcoded dimensions.
• My PR is granular and targeted to one specific feature.
• I ran npm run format in server and client directories, which automatically formats your code.
• I took a screenshot or a video and attached to this PR if there is a UI change.

[ajhollid]

There's two major issues here

1. Excess DB access
2. Header injection

that need to be addressed. There's also an architectural tweak that can keep HttpProvider less tightly coupled.

Please have a look at my comments in the code review, thanks!

[ajhollid]

We don't need a hard dependency on SettingsService here, lets just pass userAgent to the handle method instead.

[ajhollid]

There's no validation of the header, this is a pretty serious vulnerability. userAgent must be properly sanitized.

[ajhollid]

This makes a DB call on every execution of handle. This does not scale, if you had 500 monitors polling every 15s thats an extra 2000 db calls/min. User agent should be cached on startup, and the cache invalidated on update.