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

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[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]

TimescaleDB implementation is missing, and it appears that the user agent will be cleared on unrelated setting change. Can you please verify that?

[akashmannil]

Made changes @ajhollid. Please do let me know if it looks good