General engine improvements (#437)

* Add exchange manager to engine

* Several improvements for engine and friends

1) New file.Exists func
2) gRPC TLS cert expiration date check and regeneration
3) New donation var for use across the codebase
4) Use Go log package until the logger is initialised

* Add cert tests and create dir tree if it doesn't exist for file.Write

* Link up donation address to documentation tool plus minor adjustments

* Fix remaining donation addrs

* Move non-needed reload exchange funcs

* Revert accidental config_example.json changes 🕯️

* Use go logger for logging until the logger has initiliased, otherwise no output will be seen

* Link up portfolio delay val and other fixes

* Run go mod tidy after dependabot PR

* Address nitterinos

core/donation.go

@@ -0,0 +1,4 @@
+package core
+
+// BitcoinDonationAddress is the GoCryptoTrader BTC donation address
+const BitcoinDonationAddress = "bc1qk0jareu4jytc0cfrhr5wgshsq8282awpavfahc"