Refactor fee provider

[Kukks]

The fee provider ended up glued with a hardcoded factory. This PR:

• removes this glue and uses the DI to register fee provider for a network. (allows plugins to add their own fee providers, for any network
• Add a 10 second timeout to mempoolspace fee fetching as they are slow at times (many reports of downtime issues within past 24 hours)
  Fixes [Bug]: Gateway Timeout (504) on Invoice Creation because Mempool.space is down #5641.

[NicolasDorier]

I will remove the refactor you did to focus on the bug fix.

The refactor still hardcode the mempool fee provider, but on top of this limit fee providers to depends solely on bootstrap services. We should probably do something later, but for now let's just focus on fixing the bug.

On top of this, fee provider itself might require a refactoring sooner or later, as I don't think it should depends on BTCPayNetwork, probably should depends only to a cryptoCode, but still need to think about it.

Last, it doesn't fix #5641 I believe.
The issue came because mempool space was just hanging, rather than crashing this is fixed by a timeout and cancellationtoken.

Your PR is also doing some linear interpolation stuff you didn't really explained why. Looking into it.

[NicolasDorier]

• Rewrote the RandomizeByPercentage, it was bugged: This was adding or removing exactly 10% rather than picking random value on a range of +-10%
• Added tests of InterpolateBound and RandomizeByPercentage
• Rewrote the InterpolateBound to make it more concise, removing branches on the way. I believe the previous version had an issue because it was dividing a decimal with an integer.
• Removed the registration of providers in DI for the moment.
• Dispose of the CancellationTokenSource for the timeout

I will do a followup PR so that, like the rates, the fees are fetched in the background, avoiding a checkout page load hanging