Engine API: introduce exchangeCapabilities

src/engine/experimental/capabilities.md

@@ -0,0 +1,28 @@
+# Engine API -- Capabilities
+
+Specification of `engine_getCapabilities` method returning a list of Engine API methods supported by the server (execution layer client) down to a version of each method.
+
+The proposed method should become a part of [`common.md`](../common.md) document if accepted.
+
+## Methods
+
+### engine_getCapabilities
+
+*Note:* The method itself doesn't have a version suffix.
+
+#### Request
+
+* method: `engine_getCapabilities`
+* timeout: 1s
+
+#### Response
+
+`Array of string` -- Array of strings, each string is a name of a method supported by execution layer client software.
+
+#### Specification
+
+1. Client software **MUST** return a list of currently supported Engine API methods down to a version of each method. Consider the following response examples: 
+    * `["engine_newPayloadV1", "engine_newPayloadV2", ...]` -- the software currently supports `V1` and `V2` versions of `engine_newPayload` method,
+    * `["engine_newPayloadV2", "engine_newPayloadV3", ...]` -- `V1` version has been deprecated, and `V3` have been introduced with respect to the above response.
+
+2. The `engine_getCapabilities` method **MUST NOT** be returned in the response list.