feat: adding docs for payment metadata (#124)

Co-authored-by: vacwmX <vacwm01@gmail.com>
Co-authored-by: ok300 <106775972+ok300@users.noreply.github.com>

snippets/kotlin_mpp_lib/shared/src/commonMain/kotlin/com/example/kotlinmpplib/Metadata.kt

@@ -0,0 +1,57 @@
+package com.example.kotlinmpplib
+
+import breez_sdk.*
+class Metadata {
+    fun SetPaymentMetadata(sdk: BlockingBreezServices) {
+        // ANCHOR: set-payment-metadata
+        try {
+            sdk.setPaymentMetadata("target-payment-hash", """{"myCustomValue":true}""")
+        } catch (e: Exception) {
+            // Handle error
+        }
+        // ANCHOR_END: set-payment-metadata
+    }
+
+    fun FilterPaymentMetadata(sdk: BlockingBreezServices) {
+        // ANCHOR: filter-payment-metadata
+        val metadataFilters = listOf(MetadataFilter(
+          jsonPath = "myCustomValue",
+          jsonValue = "true"
+        ))
+
+        try {
+            sdk.listPayments(
+                ListPaymentsRequest(
+                    metadataFilters = metadataFilters
+                ))
+        } catch (e: Exception) {
+            // handle error
+        }
+        // ANCHOR_END: filter-payment-metadata
+    }
+
+    fun FilterPaymentMetadataString(sdk: BlockingBreezServices) {
+        // ANCHOR: filter-payment-metadata-string
+        val metadataFilters = listOf(MetadataFilter(
+          jsonPath = "customerName",
+          jsonValue = "\"Satoshi Nakamoto\""
+        ))
+        // ANCHOR_END: filter-payment-metadata-string
+    }
+
+    fun FilterPaymentMetadataObject(sdk: BlockingBreezServices) {
+        // ANCHOR: filter-payment-metadata-object
+        // This will *NOT* work
+        val _metadataFilters = listOf(MetadataFilter(
+          jsonPath = "parent.nestedArray",
+          jsonValue = "[1, 2, 3]"
+        ))
+
+        // Any of these will work
+        val metadataFilters = listOf(MetadataFilter(
+          jsonPath = "parent.nestedArray",
+          jsonValue = "[1,2,3]"
+        ))
+        // ANCHOR_END: filter-payment-metadata-object
+    }
+}