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

shared/consts.go

@@ -9,6 +9,7 @@ const (
 	MaxEntriesDBSizeBytesEnvVar      = "MAX_ENTRIES_DB_BYTES"
 	RulePolicyPath                   = "/app/enforce-policy/"
 	RulePolicyFileName               = "enforce-policy.yaml"
+	ContractFileName                 = "contract-oas.yaml"
 	GoGCEnvVar                       = "GOGC"
 	DefaultApiServerPort             = 8899
 	DebugModeEnvVar                  = "MIZU_DEBUG"