Add OAS contract monitoring support (#325)

* Add OAS contract monitoring support

* Pass the contract failure reason to UI

* Fix the issues related to contract validation

* Fix rest of the issues in the UI

* Add documentation related to contract monitoring feature

* Fix a typo in the docs

* Unmarshal to `HTTPRequestResponsePair` only if the OAS validation is enabled

* Fix an issue caused by the merge commit

* Slightly change the logic in the `validateOAS` method

Change the `contractText` value to `No Breaches` or `Breach` and make the text `white-space: nowrap`.

* Retrieve and display the failure reason for both request and response

Also display the content of the contract/OAS file in the UI.

* Display the OAS under `CONTRACT` tab with syntax highlighting

Also fix the styling in the entry feed.

* Remove `EnforcePolicyFileDeprecated` constant

* Log the other errors as well

* Get context from caller instead

* Define a type for the contract status and make its values enum-like

* Remove an unnecessary `if` statement

* Validate OAS in the CLI before passing it to Agent

* Get rid of the `github.com/ghodss/yaml` dependency in `loadOAS` by using `LoadFromData`

* Fix an artifact from the merge conflict

tap/extensions/http/matcher.go

@@ -31,7 +31,7 @@ func (matcher *requestResponseMatcher) registerRequest(ident string, request *ht
 	requestHTTPMessage := api.GenericMessage{
 		IsRequest:   true,
 		CaptureTime: captureTime,
-		Payload: HTTPPayload{
+		Payload: api.HTTPPayload{
 			Type: TypeHttpRequest,
 			Data: request,
 		},
@@ -60,7 +60,7 @@ func (matcher *requestResponseMatcher) registerResponse(ident string, response *
 	responseHTTPMessage := api.GenericMessage{
 		IsRequest:   false,
 		CaptureTime: captureTime,
-		Payload: HTTPPayload{
+		Payload: api.HTTPPayload{
 			Type: TypeHttpResponse,
 			Data: response,
 		},