backtester: custom strategy plugins (#989)

* Adds custom strategy

* docs and structure

* docs

* rn

* Documents plugins, adds custom strat config

* mini fixes. Fleshes strategy test

* docgen

* Updates plugins to allow for multiple strategies to be loaded

* docs

* docs regen

* fix doc accuracy

* why did I add the word custom?

backtester/config/examples/custom-plugin-strategy.strat

@@ -0,0 +1,71 @@
+{
+ "nickname": "ExamplePluginStrategy",
+ "goal": "To demonstrate that custom strategies can be used",
+ "strategy-settings": {
+  "name": "custom-strategy",
+  "use-simultaneous-signal-processing": false,
+  "disable-usd-tracking": false
+ },
+ "funding-settings": {
+  "use-exchange-level-funding": false
+ },
+ "currency-settings": [
+  {
+   "exchange-name": "ftx",
+   "asset": "spot",
+   "base": "BTC",
+   "quote": "USDT",
+   "spot-details": {
+    "initial-quote-funds": "1000000"
+   },
+   "buy-side": {
+    "minimum-size": "0.005",
+    "maximum-size": "2",
+    "maximum-total": "40000"
+   },
+   "sell-side": {
+    "minimum-size": "0.005",
+    "maximum-size": "2",
+    "maximum-total": "40000"
+   },
+   "min-slippage-percent": "0",
+   "max-slippage-percent": "0",
+   "maker-fee-override": "0.0002",
+   "taker-fee-override": "0.0007",
+   "maximum-holdings-ratio": "0",
+   "skip-candle-volume-fitting": false,
+   "use-exchange-order-limits": false,
+   "use-exchange-pnl-calculation": false
+  }
+ ],
+ "data-settings": {
+  "interval": 86400000000000,
+  "data-type": "candle",
+  "api-data": {
+   "start-date": "2021-08-01T00:00:00+10:00",
+   "end-date": "2021-12-01T00:00:00+11:00",
+   "inclusive-end-date": false
+  }
+ },
+ "portfolio-settings": {
+  "leverage": {
+   "can-use-leverage": false,
+   "maximum-orders-with-leverage-ratio": "0",
+   "maximum-leverage-rate": "0",
+   "maximum-collateral-leverage-rate": "0"
+  },
+  "buy-side": {
+   "minimum-size": "0.005",
+   "maximum-size": "2",
+   "maximum-total": "40000"
+  },
+  "sell-side": {
+   "minimum-size": "0.005",
+   "maximum-size": "2",
+   "maximum-total": "40000"
+  }
+ },
+ "statistic-settings": {
+  "risk-free-rate": "0.03"
+ }
+}